Contrary to what you might think, those people probably believe in the thing they posted. Just because it makes you angry, doesn’t mean it has that effect on everyone else.
I'm not aware of communities like that on Lemmy, but Reddit's got r/terriblefacebookmemes, a bunch of subs with the format r/shit____say(s), r/therightcantmeme, etc. They're unambiguously posting the content because they don't like it and want everyone to hate on it with them.
Well, laughing at something in the way you described is kind of just a different way of hating on it, is it not? Even the way you phrased, "...calling out the stupidity of shit other people say," suggests a strong dislike of the statements showcased on those subs.
Yes, but you also probably don't repost screenshots of cringeworthy anime-centric posts other people have made so that you can make fun of them with other people. There's a difference between seeing something, going, "lol that's dumb," and moving on VS seeing something, going, "lol so fucking stupid," taking a screenshot, and posting it somewhere else so you and a bunch of other people can spend time making fun of it and tearing it apart in the comments.