Mary is depicted with a crown because scripture does (Rev 12).I answer:
Revelation 12 is not about Mary, it is about the ancient church. I could quote the standard Reformed expositors on this, but perhaps you'd be more persuaded by the fact that this interpretation is, as far as I have been able to find, the unanimous consent of the fathers. Victorinus of Petau (died about A.D. 303) explained:
“And there was seen a great sign in heaven. A woman clothed with the sun, and the moon under her feet, and on her head a crown of twelve stars. And being with child, she cried out travailing, and bearing torments that she might bring forth.”] The woman clothed with the sun, and having the moon under her feet, and wearing a crown of twelve stars upon her head, and travailing in her pains, is the ancient Church of fathers, and prophets, and saints, and apostles, which had the groans and torments of its longing until it saw that Christ, the fruit of its people according to the flesh long promised to it, had taken flesh out of the selfsame people. Moreover, being clothed with the sun intimates the hope of resurrection and the glory of the promise. And the moon intimates the fall of the bodies of the saints under the obligation of death, which never can fail. For even as life is diminished, so also it is increased. Nor is the hope of those that sleep extinguished absolutely, as some think, but they have in their darkness a light such as the moon. And the crown of twelve stars signifies the choir of fathers, according to the fleshly birth, of whom Christ was to take flesh.- Victorinus, Commentary on the Apocalypse, at Revelation 12:1-2
Likewise, Hippolytus (about A.D. 170 – 236) concurs:
60. Now, concerning the tribulation of the persecution which is to fall upon the Church from the adversary, John also speaks thus: “And I saw a great and wondrous sign in heaven; a woman clothed with the sun, and the moon under her feet, and upon her head a crown of twelve stars. And she, being with child, cries, travailing in birth, and pained to be delivered. And the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born. And she brought forth a man-child, who is to rule all the nations: and the child was caught up unto God and to His throne. And the woman fled into the wilderness, where she hath the place prepared of God, that they should feed her there a thousand two hundred and threescore days. And then when the dragon saw it, he persecuted the woman which brought forth the man-child. And to the woman were given two wings of the great eagle, that she might fly into the wilderness, where she is nourished for a time, and times, and half a time, from the face of the serpent. And the serpent cast (out of his mouth water as a flood after the woman, that he might cause her to be carried away of the flood. And the earth helped the woman, and opened her mouth, and swallowed up the flood which the dragon cast) out of his mouth. And the dragon was wroth with the woman, and went to make war with the saints of her seed, which keep the commandments of God, and have the testimony of Jesus.” [Rev. xii. 1–6, etc.]- Hippolytus, Treatise on Christ and Antichrist, Sections 60-61
61. By the woman then clothed with the sun,” he meant most manifestly the Church, endued with the Father’s word, whose brightness is above the sun. And by the “moon under her feet” he referred to her being adorned, like the moon, with heavenly glory. And the words, “upon her head a crown of twelve stars,” refer to the twelve apostles by whom the Church was founded. And those, “she, being with child, cries, travailing in birth, and pained to be delivered,” mean that the Church will not cease to bear from her heart the Word that is persecuted by the unbelieving in the world. “And she brought forth,” he says, “a man-child, who is to rule all the nations;” by which is meant that the Church, always bringing forth Christ, the perfect man-child of God, who is declared to be God and man, becomes the instructor of all the nations. And the words, “her child was caught up unto God and to His throne,” signify that he who is always born of her is a heavenly king, and not an earthly; even as David also declared of old when he said, “The Lord said unto my Lord, Sit Thou at my right hand, until I make Thine enemies Thy footstool.” [Ps. cx. 1.] “And the dragon,” he says, “saw and persecuted the woman which brought forth the man-child. And to the woman were given two wings of the great eagle, that she might fly into the wilderness, where she is nourished for a time, and times, and half a time, from the face of the serpent.” [Rev. xi. 3.] That refers to the one thousand two hundred and threescore days (the half of the week) during which the tyrant is to reign and persecute the Church, which flees from city to city, and seeks concealment in the wilderness among the mountains, possessed of no other defence than the two wings of the great eagle, that is to say, the faith of Jesus Christ, who, in stretching forth His holy hands on the holy tree, unfolded two wings, the right and the left, and called to Him all who believed upon Him, and covered them as a hen her chickens. For by the mouth of Malachi also He speaks thus: “And unto you that fear my name shall the Sun of righteousness arise with healing in His wings.” [Mal. iv. 2.]
Furthermore Methodius of Olympus and Patara (about A.D. 260 – 312), explained:
John, in the course of the Apocalypse, says: [Rev. xii. 1–6.] “And there appeared a great wonder in heaven; a woman clothed with the sun, and the moon under her feet, and upon her head a crown of twelve stars: and she, being with child, cried, travailing in birth, and pained to be delivered. And there appeared another wonder in heaven; and behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads. And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born. And she brought forth a man-child, who was to rule all nations with a rod of iron: and her child was caught up unto God, and to His throne. And the woman fled into the wilderness, where she hath a place prepared of God, that they should feed her there a thousand two hundred and threescore days.” So far we have given, in brief, the history of the woman and the dragon. But to search out and explain the solution of them is beyond my powers. Nevertheless, let me venture, trusting in Him who commanded to search the Scriptures. [St. John v. 39.] If, then, you agree with this, it will not be difficult to undertake it; for you will quite pardon me, if I am unable sufficiently to explain the exact meaning of the Scripture.- Methodius, The Banquet of the Ten Virgins, Thekla (Discourse 8), Chapters 4-6
The woman who appeared in heaven clothed with the sun, and crowned with twelve stars, and having the moon for her footstool, and being with child, and travailing in birth, is certainly, according to the accurate interpretation, our mother,[Editor's note in Schaff's edition: "i.e., the Church. See p 337, note 4, infra."] O virgins, being a power by herself distinct from her children; whom the prophets, according to the aspect of their subjects, have called sometimes Jerusalem, sometimes a Bride, sometimes Mount Zion, and sometimes the Temple and Tabernacle of God. For she is the power which is desired to give light in the prophet, the Spirit crying to her: [Isa. lx. 1–4.] “Arise, shine; for thy light is come, and the glory of the Lord is risen upon thee. For, behold, the darkness shall cover the earth, and gross darkness the people: but the Lord shall arise upon thee, and His glory shall be seen upon thee. And the Gentiles shall come to thy light, and kings to the brightness of thy rising. Lift up thine eyes round about, and see; all they gather themselves together, they come to thee: thy sons shall come from far, and thy daughters shall be nursed at thy side.” It is the Church whose children shall come to her with all speed after the resurrection, running to her from all quarters. She rejoices receiving the light which never goes down, and clothed with the brightness of the Word as with a robe. For with what other more precious or honourable ornament was it becoming that the queen should be adorned, to be led as a Bride to the Lord, when she had received a garment of light, and therefore was called by the Father? Come, then, let us go forward in our discourse, and look upon this marvelous woman as upon virgins prepared for a marriage, pure and undefiled, perfect and radiating a permanent beauty, wanting nothing of the brightness of light; and instead of a dress, clothed with light itself; and instead of precious stones, her head adorned with shining stars. For instead of the clothing which we have, she had light; and for gold and brilliant stones, she had stars; but stars not such as those which are set in the invisible heaven, but better and more resplendent, so that those may rather be considered as their images and likenesses.
Now the statement that she stands upon the moon, as I consider, denotes the faith of those who are cleansed from corruption in the laver of regeneration, because the light of the moon has more resemblance to tepid water, and all moist substance is dependent upon her. The Church, then, stands upon our faith and adoption, under the figure of the moon, until the fulness of the nations come in, labouring and bringing forth natural men as spiritual men; for which reason too she is a mother. For just as a woman receiving the unformed seed of a man, within a certain time brings forth a perfect man, in the same way, one should say, does the Church conceive those who flee to the Word, and, forming them according to the likeness and form of Christ, after a certain time produce them as citizens of that blessed state. Whence it is necessary that she should stand upon the laver, bringing forth those who are washed in it. And in this way the power which she has in connection with the laver is called the moon, because the regenerate shine being renewed with a new ray, that is, a new light. Whence, also, they are by a descriptive term called newly-enlightened; the moon ever showing forth anew to them the spiritual full moon, namely, the period and the memorial of the passion, until the glory and the perfect light of the great day arise.
Gregory the Great (about A.D. 540 - 604) takes the same position:
For in Holy Scripture when the 'sun' is used figuratively, there is designated sometimes the Lord, sometimes persecution, sometimes the display of an open sight of any thing, but sometimes the understanding of the wise. For by the 'sun' the Lord is typified, as is said in the Book of Wisdom, that all the ungodly in the day of the last judgment, on knowing their own condemnation, are about to say: "We have erred from the way of truth, and the light of righteousness hath not shined unto us, and the sun rose not upon us." [Wis. 5:6] As if they plainly said: The ray of inward light has not shone on us. Whence also John says: "A woman clothed with the sun and the moon under her feet." [Rev. 12:1] For by the 'sun' is understood the illumination of truth, but by the moon, which wanes and is filled up every month, the changeableness of temporal things. But Holy Church, because she is protected with the splendour of the heavenly light, is clothed, as it were, with the sun; but, because she despises all temporal things, she tramples the moon under her feet.- Gregory the Great, Morals, Book XXXIV, at Job 41:21
Still further, in his Golden Chain, Aquinas provides the following patristic commentary on the list of the twelve apostles in Matthew 10:1-4, drawing specifically from Rabanus (about A.D. 780 - 856):
Rabanus, and cf. Tertullian, cont. Marc. iv, 13: This number is typified by many things in the Old Testament; by the twelve sons of Jacob, by the twelve princes of the children of Israel, by the twelve running springs in Helim, by the twelve stones in Aaron’s breastplate, by the twelve loaves of the shew-bread, by the twelve spies sent by Moses, by the twelve stones of which the altar was made, by the twelve stones taken out of Jordan, by the twelve oxen which bare the brazen sea. Also in the New Testament, by the twelve stars in the bride’s crown, by the twelve foundations of Jerusalem which John saw, and her twelve gates.- Rabanus, according to Aqunias, on Matt. 10:1-4
Now, I realize that modern Romanism teaches that the woman is both Mary and the Church (see, for example, sections 103-04 of Evangelium Vitae, pope John Paul II, 1995, although pope Pius X seems to have thought that it referred only or primarily to Mary, see Ad Diem Illum Laetissimum, Section 24, 1904 - see also pope Paul VI Signum Magnum, 1967, and Redemptoris Mater, pope John Paul II, 1987, and contrast the tenuous identification with Mary, with the Church being the primary referent in the Haydock's Bible Commentary, 1859 ed.), but my point is that nowhere do we see the fathers making this identification. In the 19th century we see a tenuous identification being made to Mary, and then in the 20th century we see that tenuous identification becoming the primary identification within the evermore mariolatrous religion of Rome.
The same commenter also added:
And with all due respect to Tertullian's ignorance of what priests and rulers wore crowns, priests wore a kind of a crown (Exod 39:28), people getting married wore a crown (Is. 61:10), kings of Israel wore crowns (2 Sam 12:30), saints in heaven wear crowns (Rev. 4:4) and so forth.I answer:
Exodus 39:28 mentions the priests wearing a "mitre of fine linen, and goodly bonnets of fine linen, and linen breeches of fine twined linen," and provides no reference to a crown. Not everything that covers a man's head is a crown.
Isaiah 61:10 also does not mention a crown. It states:
Isaiah 61:10 I will greatly rejoice in the LORD, my soul shall be joyful in my God; for he hath clothed me with the garments of salvation, he hath covered me with the robe of righteousness, as a bridegroom decketh himself with ornaments, and as a bride adorneth herself with her jewels.
Yes, the kings of Israel wore crowns, but the King of Spiritual Israel, the fulfilment of national Israel, is Christ.
While the elders had gold crown in Revelation 4:4, they cast those crowns before the throne of the Lord in Revelation 4:10, saying:
Revelation 4:11 Thou art worthy, O Lord, to receive glory and honour and power: for thou hast created all things, and for thy pleasure they are and were created.
So ought the prayer of all Christians to be.