I don't think the grace is because of the apparitions. Last I heard the Church has not declared them worthy of belief. (I don't believe them.) The grace comes from attending the sacraments, Mass, Eucharist and Reconciliation once you get there.
I don't know if this miracle has been declared worthy or if it is just a pious superstition. But if it has been verified as a true miracle, it is still a peripheral matter. It is pretty far down on the list of priorities. We should not need miracles. At Vatican II a hierarchy was set down and the three main things that the Catholic Church is about are the Bible, Eucharist and Prayer. Anyway my understanding of how we are to view things like this is that you can believe it if you want to but don't fixate on it. If you don't believe it that's okay, but DON'T make fun of it.
One requirement for sainthood is that miracles occur because of the saint. It could be that this is a miracle, that amoung other requirements, led to the canonization of this particular saint.