That's true about Muffy.  It's sort of the reverse for her - while Francine's usually a nice person, Muffy's usually misguided and selfish, but only on rare occasion shows hints of goodness (i.e. planting cherry trees or confessing that she cheated off of Francine).  She does seem much less likely than Francine (or anyone else, for that matter) to be able to see the error of her ways.  Either that, or she just doesn't care enough to change them. 

Of course, that's what makes Miss Muffeh so much fun to watch in action.     Well, that and all of her smashing different hairstyles...

 
Peachy icing on a bundt cake, dammit!