I don't think anyone could have missed it, but the webcomic Kevin and Kell lives by this trend. In that universe, catching, killing, and eating other sentient members of your society is not just acceptable, it is considered the right and natural way of the world. However, if that ever looks like it is about to happen to anyone in the protagonist family's inner circle of friends it is treated like a genuinely bad thing -- not just from the perspective of you the reader (where, 'oh hell, yes I know this is a crapsack world, but that doesn't mean it should happen to my favorite character' is somewhat reasonable), but by the characters in the universe (where this is supposed to be the way of the world).
There are plenty of 80s toy-media properties (Transfromers, GI Joe, etc.) where the villain characters are often much more 'cool' than the heroes -- GI Joe notably, in that the villains were in bright contrasting color combos like blue-red or gold-green (or a silver or gold head), while most of the heroes were of interchangeable camo/drab looks. I bet that had some impact.
Looney Tunes (and similar animated shorts by other companies, such as Hanna Barbara's Tom and Jerry) pretty much treated being cagey and crafty as virtues in and of themselves. The 'villains,' however, weren't really mentally challenged, just obtuse (a rabbit in a dress is indistinguishable from a gorgeous woman), monofocused (a Yosemite Sam incarnation defending the Mason Dixon line 80+ years after the end of the civil war), and absurdly gullible (Elmer Fudd buying that the gold tooth he got in a fight was Bugs', even though he could have felt that his own gold tooth had come out if he put any thought into it). The too-stupid-to-live characters (usually one-off characters, usually some play on Of Mice and Men's Lenny or a Lou Costello expy) usually were portrayed sympathetically. In real life, too, we tend to treat mildly unintelligent (seeming) individuals (think Jessica Simpson tuna confusion) as acceptable targets, but those that reach the level of disability are (hopefully) not acceptable targets. Not really sure that says good things about our society that we have a range of intelligence that is acceptable laugh fodder (but not enough to garner sympathy), but I don't think LT deviates particularly from that norm. Mind you, I'm thinking of the ones that got shown in the 70s and early 80s, so maybe the worst offenders had already been culled from circulation (although thinking back to what was left in...).