Despite the thumbs-down, I approve of this picture.
Timmy was designed to be a (clearly) handicapped character who would be treated just like everyone else.
Instead of limiting him to only positive portrayals to avoid offending anyone, he was allows actual development and flaws.
Obviously, a show for little girls isn't trying to be as controversial as South Park, but accepting the disabled as real people, not unrealistic paragons, is a healthy lesson to learn.
(not to say that Derpy is actually disabled, just that many of the complainers assumed that she was.)