I think I have an idea. When you post/comment, there should be a checkmark for “are you being serious?”. The default value should be chosen on a sub-per-sub basis, so all comments non-serious by default on, say, c/memes, and serious on c/news, for example. Then that information should be hidden unless you downvote or reply to a comment/post of the opposite seriousness to the default of the sub you’re in (I guess there could also be an option to see the warning always or on demand).
I think I should post this properly somewhere but idk where…