It's not about forming opinions though - it's about deciding what order to sort posts. Sorting by date / view count / upvote count is pretty crap. We only get away with it now because there aren't many people posting. That's changing.
Pretty much all social networks these days have some kind of "karma" like score, though it often isn't visible to anyone. Obviously in the fediverse it would be visible (though maybe not activley promoted?).
The best karma implementation that I know of is the one on Stack Exchange, though I'd like it more if it wasn't so easy to have a high score just by having been on the network for a long time (my score over there is ridiculously high for someone who barely participates these days).