I'm going to put in a complaint about a coffee shop worker
Why would you complain about it?
I dislike him and feel powerful doing that.
If they provided poor service, you have every right to complain. They should be reprimanded for that.
But I'm also worried about looking mean if I do that. Should I just go to the supervisor secretly?
Would that make any difference? If the service was that bad then it is something you should bring up either way. If you don't then the service everywhere will stay bad.