Is it normal that my ass sweats

Condition often refered to as Swamp Ass.

I assume you mean your ass crack. That's normal. Perhaps try a thicker underwear made for athletes too soak up sweat. I am sure they are available.

Throw out the synthetic underpants and get cotton ones. By the way, normal for any part of the body to sweat if it's hot, that's just how bodies function to cool down

Same.