I learned from the little woman that ginger bashing is much more common in europe (she's from the Netherlands, and has naturally dark red hair). Maybe, as previously said, it has something to do with hating on the irish, I dunno. Here in the states, at least by my experience, it's far less commonplace. In fact, I'd never actually heard them referred to as "gingers" (as opposed to having "ginger hair") until that episode of south park.
That's not to say that they don't have stereotypes. Sexual aggressiveness. Tempers. Sunburns. Health problems. That sort of thing.