A single downvote is not an expression of a community norm. It is an expression by a single person that there was something, and it could be pretty much anything, about your post that that one person did not like.
It’s not just one person though. Having −1 points also means that nobody else thought it deserves more than that, or at least it’s not worth their effort to vote it back up to 0. So if you have reason to think the comment has been read by more than a few people after it was downvoted, even −1 points does reflect the community judgement to some extent.
Generally, the only types of comments that are below −3 that I upvote are ones which I think add a perspective to the conversation which should be there but should have a different proponent. It’s rare that I find a comment at less than −3 which I would fully endorse (but I have my settings set to display all comments).
It’s not just one person though. Having −1 points also means that nobody else thought it deserves more than that, or at least it’s not worth their effort to vote it back up to 0. So if you have reason to think the comment has been read by more than a few people after it was downvoted, even −1 points does reflect the community judgement to some extent.
Indeed, my quality threshold to upvote comments at −1 is much lower than my quality threshold to upvote comments at 0.
What function describes your threshold as the negative values go below −1?
Generally, the only types of comments that are below −3 that I upvote are ones which I think add a perspective to the conversation which should be there but should have a different proponent. It’s rare that I find a comment at less than −3 which I would fully endorse (but I have my settings set to display all comments).