Ass so mean that I’m always on her bad side
He’s using wordplay to say that he likes looking at her from behind.
To be on someone’s “bad side” is an idiom for when someone is criticizing you harshly.
In this case, her bad side is her “backside”. Get it?
To help improve the meaning of these lyrics, visit "She Don't Put It Down" by Joe Budden (Ft. Lil Wayne & Tank) Lyrics and leave a comment on the lyrics box