I just looked at the top 10 posts for the last month, and 10 newest posts, and they're really not that negative. I'd say it's a third quite positive, a third fediverse / tech, and a third quite negative.
But humans perceive negative information much more strongly (at least according to some studies) so even though they're evenly split, the negative ones might stand out and be more salient. This month's top ten included "Why does it feel like evil is winning in the world?" (negative) versus "what's your favourite local dish from your part of the world? (positive). The positive one could feel small and trivial while the negative one feels big, serious and apocalyptic - so of course it stands out more.
As well as salience, people are more likely to post about a problem than a solution (it is asklemmy after all), so it's really going to be problems, big and small, or fun / silly questions like the local food one. Even more neutral questions about advice are often revolving around a negative problem. I've just posted asking about good alternatives to discord (fairly neutral I think?) and I'm hoping for positive "this other thing is great!" responses, but I'm kinda asking because I'm sick of enshitification (which is negative).
The other possibility is that the new and top rankings are not representative. Perhaps posts are mostly mixed but the ones that get popular are more negative, but the occasional popular poistive one is such a pleasant surprise that it ends up making it to the top ten.
