It's best if you don't bitch about it, because pretty much everyone did, and I don't think anyone who works on the show will change their minds.
Besides that was supposed to be a type of monster.
I don't see you complaining about the tons of villains that are deformed/have mental problems. From Spiderman, Superman, etc.
It's not like they went to the store and saw little Jimmy with a dent in his head and said "Ew wtf is that.".
Besides MLP and TMNT are two different shows. MLP is for little girls, which is why everyone reacted so hard.
Also there have been characters from shows that are just completely stupid and is a main character. (or side)
Cheese from Fosters.
Ed from Ed Edd n Eddy.
Patrick Star from Spongebob Squarepants
Peter Griffin. (Who is actually mentally retarded, like seriously theres an episode with a song and everything)
Homer Simpson.
The list goes on.
Also "Derp".
-facepalm-
So many people like you have said, "Derp doesn't mean anything relating to being mentally handicapped".