Is it okay if I tell my fiance his mother is a sack of shit?
Do you have a problem with your fiance's mother?
Yes, she was critical to my actions during upbringing.
It's totally natural and normal for your fiance's mom to do things differently than what you're used to, but it doesn't mean they're wrong. It just means you're different, so work with them and try to come to a mutual understanding.
I can't deal with her irrational expectations anymore.
Your going to be seeing a lot more of their mom since your getting married, so it would be best to try to find some common ground starting now. Try to treat her mom as you would your own and she could start to warm up to you.