I do think it feels insular/hostile, though my take is different than yours. Yes, I've had some folks nitpick posts and one up, though I imagine you get that in many similar social media sites -- there're reasons for the running joke that in order to get a correct answer online, you gotta post a question as a woman, and confidently answer it wrong on a male account, and then people will pile on with correct answers and judgey-ments.

Many of the sites that seem to get posted to lemmy are on VERY fringey locations, locations that often seem highly likely to be hosting malicious content. There's also far fewer people posting topics, with a very small group of users having like 10k+ posts per year. So it feels a lot more 'concentrated', especially on particular subjects, due to those posters' bias': if you're an aggressive poster, it's fairly easy to astroturf lemmy. Which feels insular / bubbly.

Adding to that, Lemmy has mods who are just as heavy-handed as on Reddit or any other heavily moderated setup -- though lemmy'll moderate things in a far more 'progressive' friendly way than most alternatives these days. Pejoratives are allowed for some groups, but not others. Even calling that sort of thing out in more explicit terms, though still in a neutral tone, can get you banned from many communities on Lemmy.