HN comments can still be downvoted (if you have enough karma) so arguably you can still get signal quality from the sum of votes.
Especially since in most cases, you can simply comment instead of downvote.
Downvotes on HN comments aren't a pure quality signal. HN's culture is that downvoting something just because you disagree with it is perfectly cromulent behaviour, as established by PG himself:
https://news.ycombinator.com/item?id=117171
https://news.ycombinator.com/item?id=658691
Honestly, I'm not a huge fan of that decision, but I didn't start this site.
If my comments are highly upvoted, I've hit upon a truth or realization with which many people agree. If my comments are downvoted, I've also likely hit upon a truth or a realization, but one with which many people are deeply uncomfortable.
It's up to my own powers of discernment and wisdom to determine if people are all wrong and the prevailing sentiment is incorrect, or if it's me who's out in left field and is significantly off-base. We will never engineer a button to press to give us this insight.
Also, at a sample point of one, I make an effort to never downvote because of disagreement. I don't behave that way on reddit. Something about herd mentality perhaps.
It could be that people who agree upvote without replying, people who disagree reply to argue against it. HN discourages non-substantive replies; if you agree with everything someone says, it can be hard to generate a substantive reply to it. (In practice, occasional non-substantive agreeing replies are tolerated, but people who do it more than occasionally often encounter some pushback.)