Since Sister Faustina has been canonized, meaning that her claims have been approved by the Church, I will believe them. She would not be St. Faustina if they were heretical as you claim.
There are numerous saints on the calendar who have made heretical statements in the past.
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.