@Mad-Mutt I think the part of the difference with Snips and Snails IS that they are kids, there can always be the argument that "they'll grow out of it eventually". Also while they are a bit on the "slow" side that really isn't exactly the same thing as being "minimally exceptional". Plus of course Snips and Snails are sort of bad guys; it's OK for BAD guys to be dumb.
@Kusang. It interesting you bring up Goofy since, in some of Disney's iterations, he is ALSO supposed to be a single parent (Goof Troop/ The Goofy Movies, of course but also a few of the older ones don't have Mrs. Goof either)
Stimpy I think got by a lot because it was a character piece. Stimpy wasn't just some random dumb guy, he was a Stooge (that's what the shows creators said was their inspiration deciding it would be fun to put a caricature of Larry Fine (of the Three Stooges) with a caricature of Peter Lorre). Ren thinks Stimpy is an idiot but that's part of the joke, because of his ego, he thinks EVERYONE is an idiot.
Patrick gets away with it by contrast. Because he is so dimwitted, he makes SpongeBob look bright by comparison and comparatively normal (without him SpongeBob would probably be seen a lot less favorably than he is, both within the show and by the watchers.
I think it really all DOES boil down to the voice in the cameo. If the voice actor had KNOWN Derpy was a girl, and voiced her as such (the way she had been the last time she spoke) I think there would have not been as much backlash. Derp came off as not merely "a little different" but as "needs assisted living retarded" and (at least to me) hearing impaired as well. Also as much as I hate to say it maybe we really should have accepted their proposed choice of "Ditzy Doo" as her name. Though to me at least, calling a female character (especially one who is technically blonde) "ditzy" is probably almost as insulting as calling her a derp, if not more so
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