It's been a couple years since and I still don't know the full details of the "derpy controversy" TV tropes has no information beyond that it was chaos worthy of discord.
I'm sure Hasbro and DHX know the full story, regarding what complaints there were and what exactly they did to try and keep the innocent pegasus from offending anyone ever again.
TV Tropes just has the basic gist that most Derpy fans know. :/