Sometimes one may knowingly add a comment that should be tagged as one of those, and sometimes I see people say (me included) things like "BTW this should be tagged <as such>." Maybe allowing a user to tag their own comment proactively with these three tags would be useful?
Edit: My main focus is the offtopic tag because I think that it's not necessarily bad or low-quality. Partially off-topic content can be very interesting and useful. Maybe I'm misunderstanding the use of that tag, was it intended for completely off-topic stuff in the first place?
Edit 2: I've opened an issue on Tildes Gitlab for this.