alt Hacker NewsI don't have a problem with people doing that as long as they don't pretend that every other commenter holds the same contrary opinion and that the downvotes indicate they're too sensitive to discuss such things, or other similar rationalizations. If you want to leave some drive-by snark without rationalizing it as being about other people, it's not my favorite kind of comment but I'm not going to object to it either.
The downvoting functionality here and in other forums can mean many things. It isn't a precise thing. If it is precise I'm not finding a clear definition. It can mean I disagree, this is boring, this is false, this made me sad, I don't like reading this, I don't like this user, I'm tired, etc.
One plausible interpretation of a downvote without a comment is drive-by snark without rationalizing.