sadly no matter how harmless the joke, SOMEONE, SOMEWHERE will get offended no matter what you do. I do not find the therm "derp" offensive, it's describing more of a clumsy person, it's when you have those "derp" moments, where you do something really clumsy, not making fun or referencing handicapped people.
So Keep Derpy Derpy! I have signed this petition for a good cause, either they'll keep it and piss off a couple of conservative moms (have not heard "helicopter mom" before) or change it and piss off hundreds of thousands of fans, fans that basically made the character from the small background pony.
FYI: I've seen a lot of mentally handicapped people supporting Derpy, not being offended at all, and saying they can decide what they are offended at themselves.
I hope they keep it, for the fans sake, a couple of overly sensitive moms hopefully can't change something like that, the little girls don't care, the bronies disagree with the complaints, mentally handicapped people are not offended, I don't see how changing Derpy's name and appearance is going anywhere.