Richard Seymour talks about the dangers of social media platforms in The Twittering Machine. This isn't trifling. It leads to real harm. One factor is downvoting, as you say.

It's so easy to click it nonchalantly because you disagree. But to the person who only sees potentially thousands of downvotes, it can hit their mental health hard. Maybe this depends on the kind of content, but idk. I'd expect someone to feel more down if they're being genuine, giving advice about a hobby, something else that is personal in some way. This probably includes politics, as politics reflect values and can be wrapped up in a how someone sees themselves as a 'good person'; which makes it difficult if you're then made to think you're wildly off the mark and, possibly, a 'bad person'.

If downvotes are getting you down, you may want to reassess how you engage with social media. The best antidote is logging off for a while. Touch grass, get some fresh air. Don't engage with the communities that give you grief.

More broadly, yes, Reddit is toxic af. More so than many other places. Twitter can rival it at times but the format can take the sting out of some of the pile-ons. Some of that toxicity has come to the fediverse.

It's been especially noticeable since the API thing as new users have just seen the fediverse as a Reddit alternative. As in, Reddit but on a different server with a different brand name. Many seem not to have cared whether the culture is the same here. Or they haven't realised that an instance might have instance-wide rules (it's not only community rules that one must understand).

Up until the API thing, Redditors occasionally brought Reddit toxicity with them, but they were outnumbered and tended to change their ways or disappear. Now, they might get support for that kind of behaviour from other new users.

In fact, if you look at some of the user accounts of the people chastising you – for (a) being concerned out yours and others' wellbeing and (b) for wanting to improve/maintain a positive culture – or dismissing your concerns, they're new accounts, probably come from Reddit recently with some of that toxicity. Maybe this would have happened two months ago? I can't remember anything like it around here, though.

Here's hoping the Reddit culture a swift end.