I truly don't think any series can be over cosplayed. If someone likes the anime and wants to cosplay from it, let them. I guess, the only thing that bothers me is when they cosplay from a series just because it's popular, or because it's easy *coughcoughDeathnotecough*
I mean, I'll be Roxas from Kingdom Hearts, and even though he is over done, I'm still gonna do him for I think he's awesome. Of course, I'm also cosplaying a character from a series that's not to well known.