Offensive user names (and Gravatars) are something we should not tolerate, in the same way we should not do so for content of posts. "Offensive" is a little hard to pin down (different things are offensive to different people), but if you personally find something like this offensive then let us know.
Now, you can't directly flag a user for moderator attention, but if you feel the need to tell us about something you believe is wrong about another user's account or profile then please just flag any post they own and use the
it needs ♦ moderator attention →
other option to explain the problem. Even just "user's name is offensive" would probably suffice.
Failing that, a meta post (like this one) or even a quick shout in chat will do the job.
But generally, flags are preferred where possible because of potential privacy issues - you don't want some troll targeting you for calling them out.
If necessary, the moderators can then contact the user, ask them to change it and maybe take direct action if it becomes necessary.
As an aside, that particular user seems to have created an unregistered account just to post an offensive snide remark, both they and the post have been removed.