Downvotes = “I disagree” or “this is bad and you should feel bad”?

I think this is very close to the most solid answer possible. Like

This is Bad content

I agree completely with this bit. Downvotes are inherently subjective, as is the concept of Bad content. But to make a choice of what to downvote, someone has to identify something worth deeming downvotable, and screw it, that's a good way to deacribe what the majority of what falls under that umbrella.

The next bit is where I'd make a correction.

which I want others to see less of

You can't unsee that bad content, it's too late. And you can't guarantee that downvoting will dissuade its continued presence. The only correlation between the two involves an expected emotional attachment between the posters of the bad content and their scoring outcome, and that's not always here nor there. Bad content posters can be persistent.

But downvoting it has an immediate effect on the visibility of the Bad content for other people. It also labels that content. Doing so, puts it away from other people's eyes, and tells others that someone thinks it should be put away. Maybe they'll come to agree or disagree with that downvote, maybe it'll lead to you seeing less content. Also no guarantee. But that immediate effect, the visibility and the score, can not be taken away.

In either scenario, it's a communication tool. It may relate to your wishes for content, but mechanically, its impact is felt by a third party.