I've rarely seen such a difference in looks between actress & character
I don't just mean how good or bad she looks, although there is that (Eden Sher looks significantly better than Sue Heck).
I mean, when I see photos of the actress and photos of the character, they look like two different people. Of course, you can tell it's the same person when you look more carefully, but without knowing, I wouldn't assume they are the same person.
I had watched an episode of The Middle (pretty good show - only seen a couple of episodes). She stood out because she was very good in the role, so I looked her up. I expected to see a plain-looking teenager. I did not expect to see an attractive twentysomething (who looks like a twentysomething).
You hear about people changing their looks for roles, but it's usually a one-off thing. I guess it's like America Ferrera and Ugly Betty, but this is a lot more subtle. It's harder to make someone look plain than it is to make them look ugly. Sue Heck is just so plain, and Eden Sher is not.