people are mad over derpy?
It's how they view the word "derp". From what I remember, it was originally (and still) used to reference having done something stupid or having screwed up, like saying "Oops, I derp'ed". However, many have taken the meaning to signify a disabled person, and when they think of that in terms of Derpy, they think Derpy is an offensive slap to disabled people.
Either way, people are too easily offended nowadays. In fact, some people
want to be offended so they can voice themselves, or put themselves into situations where they can be easily offended.