Brady gets called Marsha by the haters because he's named Brady, and it's a reference to a TV sitcom. Let's not pretend that the 5-head doesn't have 'mean' things said about him. Not everything has some deep meaning.
Well of course it's because of the show. But the key thing is they are calling him a girl's name, with the implication he isn't tough and whines like a girl. I don't think I've ever heard Peyton haters call him a girl.