There seems to be a misunderstanding about the nature of canonization inherent in the question.
Canonization (of saints) is the process by which any soul is declared officially to be in heaven, but it does not mean that those not yet declared to be in heaven either a) are not, or b) are not worthy of heaven—even if not there yet (e.g. are still living).
The sainthood (state of being worthy to enter heaven at any given time, including being there) of those who are canonized, are normatively inferred by an official investigation by the Church into the reality and recognition of miraculous answers to prayers resultant from invoking the intercession of the given person.
Sainthood does not necessarily begin when one is in heaven. Indeed those who have no remaining temporal debt for forgiven sins (with no purgatory to endure before entering paradise) are already saints. They have made sufficient reparation for the wrongs they did in so doing attaining perfection "in this life" and not by compulsion "in the next" (Mt 12:32) in purgatory. They have not so much as a "penny" remaining (Mt 5:26) to repay God in any sense, since they made it their duty to do so while in earth, by His grace. It is rare for someone to depart this world in such a state, or to find someone so willing to sacrifice comfort in this world! God is always willing (Jude 1:24), but not always the soul (2 Cor 6:1) to whom He provides His help/grace.
Accordingly, Mary, who never knew sin, as His sacred Ark, was a saint from conception and remains so, and did not become a saint only when she entered the glory of her Son:
Psalm 132:8 Arise, O Lord, into thy resting place: thou and the ark, which thou hast sanctified.
† Sancta Maria ora pro nobis! †