So apparently they decided to bowdlerize Derpy a little bit. That's... Well, that's fair enough, but since the cat's out of the bag already I think a more elegant plan would have been to immediately write a little exchange into a future episode which would retroactively make her seem more legit in the original scene (Sheepishly showing ideas for a new town hall to build or something), and let the voice drift to more of a surfer or Tommy Chong vibe (which is always what *I* imagined anyway).
There's something kinda untouchable about a character who was created from an animation gaffe and so whose special talent is, naturally enough, fucking up. And c'mon, "derp" just means, like, "brainfart" now. When I was real little "duh" was considered verboten, and now look.
In any case, it's always interesting to see exchanges such as creating a character between media and its audience; it's like TV's version a rock show where the band and crowd really get in synch.
Edit: Actually, I bet that's a big part of the reason people are so especially sad about it - They put together this whole character among themselves that got to the point of public acknowledgement and now they're all like, "Derpy was *ours*! Back off, media cops!" on top of the sting of being accused of collectively having a go at the cognitively disabled. When I first saw that scene (actually the second time, once I'd stopped going "eeeeee!") I *did* wonder if someone was gonna be upset, especially with the poor enunciation, but now she doesn't sound like *any* manner of doofus, and like entropy, loopiness in a comedy must always increase.