You can flag posts at 15 reputation. Flagging comments is not mentioned in the privileges list, so it's either the same or possibly even as low as 1. Or it may come with the "comment everywhere" privilege you get at 50 rep.
If you can flag comments, then it will be via the little grey flag on the left of each comment. The up-arrow is to vote agreement with the comment, rather than add an explicit "Me too". [These used to be "mystery meat", appearing only on mouseover; it looks like that's changed.]
Do note that comments are brief and can be brusque. That's not rudeness. There are some guidelines on flags, which — although they probably need updating after five years — still hold true. Insults are rude and flagworthy, and moderators will deal with those; a criticism which is simply "directly expressed" is a judgement call. Moderators try to be consistent, but we're only human; and some have thicker skins than others.
If you can't flag, then I would suggest comments really aren't worth complaining about. You may find that others flag them (and comments can disappear with a fairly low number of flags) and too many flags raised for any particular user also gets brought to moderators' attention by the system. But the easy answer is just to ignore any you don't like.