I get more replies like "this is over my head" from Mastodonsters whereas I think most of you might think you'd look stupid if you said something like that.

Not sure that’s the reason. Or at least all of it. HN tends to value substantive posts and someone just saying “I don’t understand this” doesn’t add to the discussion and would likely be downvoted. On the other hand, I have seen people here say they don’t understand specific bits of a post. Those are actionable and advance the discussion, and tend to be upvoted and get replies.