Mary is referred to as queen not just of heaven, but also of peace, of the angels, of all Christians, among many other things.
Because Mary is the mother of Christ the King, it is logical that she would be called Queen. (A modern example would be the mother of Queen Elizabeth II of England, who was known as the Queen Mother.)
Pope Pius XII affirmed this in his encyclical Ad Caeli Reginam when he wrote, “according to ancient tradition and the sacred liturgy the main principle on which the royal dignity of Mary rests is without doubt her Divine Motherhood.” St. Alphonsus Ligouri, said, “Because the virgin Mary was raised to such a lofty dignity as to be the mother of the King of kings, it is deservedly and by every right that the Church has honored her with the title of ‘Queen.’”
Mary played such an important role in bringing Christ the Redeemer into the world. Certainly, only Jesus Christ, the God-Man, is King; but Mary, too, as Mother of the Jesus, is given the title of Queen , not meaning she is on equal footing with God, but reflects instead how intimately connected she was, and is, to her son and his mission.
Dear Mary, Queen of Heaven, pray for us.