They might be a 'Me Too' article. They might say something painfully obvious. They don't detract from the discussion, but they don't necessarily significantly add to it. They are the comments that require the most attention from the moderators, and they also represent the bulk of the comments. Bad Comments are flamebait.
Bad comments have nothing to do with the article they are attached to.
They call someone names. They ridicule someone for having a different opinion without backing it up with anything more tangible than strong words. Bad comments are repeats of something said 15 times already making it quite apparent that the writer didn't read the previous comments. They use foul language. They are hard to read or just don't make any sense.