mikasa isn't perfect, she loses control of her emotions multiple times and she does have character development, perhaps not as much as she should have but she does have a backstory and interesting conflict.
levi on the other hand not so much.
Levi is much better than Mikasa.
He comes off as emotionless but he has to be when he's the corporal of the die-in-one-chapter squad.
He also doesn't let his emotions get to him as much because he can see how bad an idea attempting to capture Annie was.
Mikasa on the other hand, is so obssesed with getting the female titan, even though it's quite obvious that they've gotten their top priority back.