There are three possibilities for visions, locutions, etc:
(a) Supernatural heavenly origin
(b) Supernatural demonic origin
(c) Natural origin (i.e., not a real vision)
The Church carefully studies any person claiming to be receiving visions or locutionsl. They study the person, their life, the content of the visions/messages, etc.
This is a lengthy and thorough process. During that time, it is typical for the events to be supressed-- the person is not to talk about them publicly, etc. That is to ensure none of the faithful are led astray.
If, after thorough research, the Church finds these visions/messages worthy of belief they will declare them so. This is rare. Many, many people make claims of this nature and most are condemned as false or demonic in origin.
It is **inappropriate **to make statements that St. Faustina was “crazy” or a schizophrenic. She is a canonized Saint of the Church. Pope John Paul II declared the Sunday after Easter as Divine Mercy Sunday. The Divine Mercy Chaplet is an official devotional of the Church as is the Divine Mercy painting.
No Catholic is required to believe in private revelation or to read about them, etc. However, Divine Mercy is now a feast day of the Church, which elevates it beyond simply a private devotion.