If Christ were not King, Mary would not be Queen, as all her prerogatives and titles depend upon her being the Mother of the Son of God made Man for our salvation.
Christ succeeded on earth to the throne of King David. However, unlike David, Christ’s kingdom is not an earthly one, but a spiritual and eternal kingdom of truth, righteousness, and peace (Jn 16:36-37, Heb 7:2, Rev 19:6). In that Kingdom Mary sits as Queen, as queen mothers (gebirah) did in ancient Israel before her (1 Kings 2: 13-21). Their office often determined who would be king, how the kingdom would be ruled, and how the kingly favors would be given (cf. Jer. 13:18).
Mary’s queenly and maternal reign is in the spiritual realm, however. Her raiment is charity and virtue surpassing all other creatures, and her favors are the graces of her Divine Son to be given to her spiritual children in accordance with His will.
In his 1954 encyclical on Mary’s Queenship, Ad Caeli Reginam, Pope Pius XII wrote,
From the earliest ages of the Catholic church a Christian people, whether in time of triumph or more especially in time of crisis, has addressed prayers of petition and hymns of praise and veneration to the Queen of Heaven. And never has that hope wavered which they placed in the Mother of the Divine King, Jesus Christ; nor has that faith ever failed by which we are taught that Mary, the Virgin Mother of God, reigns with a mother's solicitude over the entire world, just as she is crowned in heavenly blessedness with the glory of a Queen.