If something is sweatier, it has more sweat on it or in it.
Sweaty things can feel wet, sticky, and not very clean.
Examples:
After running, my shirt is sweatier than before.
The room gets sweatier when there are a lot of people in it.
She becomes sweatier when she exercises.
The dog's tongue is sweatier than mine!
In the summer, it gets even sweatier outside.
The gym is always sweatier than my house.