Infallible teachings and infallible “dogmatic facts” have no possibility of error. Non-infallible teachings and non-infallible decisions of the Church on discipline and matters of prudential judgments have no possibility of grave error. And since the Church is indefectible, and is founded on Peter and his successors, the Popes and the Ecumenical Councils approved by the Popes can never err to a grave extent. Their use of the keys over doctrine and discipline is always at least non-infallible and free from every grave error.
Therefore, if canonizations do not fall under infallibility, then canonizations of Saints by the Roman Pontiff are at least non-infallible, so that they cannot err gravely. A canonized person might be less holy than is generally though; but such a person cannot be evil or faithless or entirely unworthy of imitation. For such a grave error in canonizing an evil person or a heretic or schismatic would harm the indefectibility of the Church.
Canonizations are at least non-infallible and therefore cannot err gravely.