Adding more to 2 previous points I've made:
(On that one about Tumblr and Deviantart and sincerity).
I think people treat sincerity as an "aesthetic" rather than a state of being.
Because humans are all different from each other, so sincerity means different things to people.
But the kind of people that say "it's ok to be cringe" might associate sincerity with "cringy things".
This allows someone to still be insincere but simply add basic elements associated with cringe, to give the idea they're being sincere instead of exploiting basic notions people have.
Because it's like: When George Carlin does angry rants where he goes "YOU KNOW WHAT, MOTHER FUCKER?", is he being insincere because he's not coming off as polite and very innocent?
It's also like saying David Lynch's works aren't sincere because he didn't add very colorfull furry characters or poorly drawn anime girls in Twin Peaks or something.
It's similar to game journos thinking any game that isn't a cinematic moviegame with Troy Baker voicing a hairy white guy isn't good enough.
It's also why I don't really like that Sonic "I WANT SHORTER GAMES" image: It's an opinion someone believes it but portrayed through ironic meme culture.
With art and paintings in general, some people associate the "post modern" art playing with shapes and all with "fake bullshit, probably a scam" compared to more detailed and creative paintings.
I notice some rightwingers are more likely to come off as racist, which may be a sign of the fact that Trump won and they're going on a "we won too hard lol" way to the point they can't do the "look at the basic black guy that pisses off the left, who's the real racist?" thing anymore.
Meanwhile, leftwingers adopt political correctness when at some point, even Marxists thought political correctness wasn't really a good thing (And even a movement like the Nazis were labelled that).