Big Patty was never really mean at all.
She was Actually pretty nice to people who take the time to get to know her and attempt to be her friend.
This is noticeable in the episode "Polishing Rhonda" when she graduates first in the class and has to coach Rhonda.
She's only mean when being disrespected by others or when people judge her by her appearance and whatnot. Ex, Wanting to beat up Helga only because she was making mean jokes about her looks. ( which was funny but highly disrespectful).