There's another way that we can arrive at the conclusion too - which is the papist notion that Mary, as "Queen of Heaven," is the queen of Mercy (whereas Christ is not the King of Mercy, but the King of Justice, in the description below ... though he certainly is called the "King of Mercy" elsewhere in Roman Catholic writings).
The kingdom of God consisting of justice and mercy, the Lord has divided it; he has reserved the kingdom of justice for himself, and he has granted the kingdom of mercy to Mary, ordaining that all the mercies which are dispensed to men should pass through the hands of Mary, and should be bestowed according to her good pleasure. St. Thomas confirms this in his preface to the Canonical Epistles; saying that the holy virgin, when she conceived the Divine Word in her womb, and brought him forth, obtained the half of the kingdom of God by becoming Queen of Mercy, Jesus Christ remaining King of Justice.As reported here (link), and essentially confirmed here (link).
For comparison (link).
This title, "Queen of Mercy," is apparently even part of the ordinary (i.e. not extraordinary) teachings of the Vatican, for it can be found in the document, Marialis Cultus (link), subtitled: "For the Right Ordering and Development of Devotion to the Blessed Virgin Mary." In that document, it is written: "It is also important to note how the Church expresses in various effective attitudes of devotion the many relationships that bind her to Mary: ... in loving service, when she sees in the humble handmaid of the Lord the queen of mercy and the mother of grace ... ." (Marialis Cultus, paragraph 22, emphasis added)
Thus, we can see that this concept of Mary's alleged Queenship of Mercy is actually the standard teaching of the modern Roman Catholic church. One could argue that Marialis Cultis is written in such a way that it qualify as an ex cathedra proclamation under the standard enunciated by the first Vatican council, although I recognize that modern Roman Catholics would almost to a man not recognize it as such.
But this is not the teaching of Scripture. Scripture states that "the Lord is very pitiful, and of tender mercy." (James 5:11) Likewise, it tells us that God is "rich in mercy" (Ephesian 2:4). Furthermore, it clearly indicates that it is by his compassions and mercies that we are saved: "It is of the LORD'S mercies that we are not consumed, because his compassions fail not. " (Lamentations 3:22)
And Scripture also teaches that God is sovereign in his mercy: "I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion." (Romans 9:15) So then, it is purely the invention of the imaginations of men's hearts to elevate Mary from the handmaid of the Lord (Luke 1:38) to the Queen of Mercy. Let us turn instead and pray to God alone, beseeching him for Mercy who is the Merciful God (Deuteronomy 4:31).
For it is written: "for the LORD your God is gracious and merciful, and will not turn away his face from you, if ye return unto him." (2 Chronicles 30:9) Therefore, repent of yours sins and believe on the Lord Jesus Christ alone for salvation.
UPDATE: Updated to reflect the fact that despite the division discussed above, Roman Catholics elsewhere do call Christ the "King of Mercy," since - based on a single comment I received, it appears that this was not clear from the original post. Also, despite criticism to the contrary from the same commentator, Mary is not only described by Catholic authors as the Queen of Heaven, but also the Queen of Hell: "Mary, Queen of heaven, is also Queen of hell; the devils themselves, bend under the yoke of her sovereignty ..." (source).