There are numerous saints on the calendar who have made heretical statements in the past.
I would say her canonization was a mistake.
The pope is a monarch and may do as he likes.
I believe most orthodox Catholic theologians agree that canonizations are infallible Papal acts. (Ludwig Ott, for example, says so on p. 299 of the English edition of "Fundamentals of Catholic Dogma".)
One can say that her canonization was perhaps prudentially unwise, but it would not seem to be within the bounds of Catholic orthodoxy to assert that St. Faustina is not enjoying the beatific vision, as well as perhaps the sin of rash judgement.
So I assume you meant the former, not the latter. ;-)
Of course, her canonization does not make her private revelations any more (or less) authoritative. It's a claim about the state of her soul and its salvation, not about her private revelations.
However, if her revelations are not from Christ, she was either seriously misled (and perhaps in frequent contact with a demonic entity) or lying. Neither status is especially consistent with the heroic sanctity characteristic of a canonized saint.
Her confessors and superiors at the time, who were holy priests and nuns, came to the conclusion that her revelations were genuine. That, along with the later approval of the Church for her cause for sainthood, is sufficient evidence for me.