I've often felt I hadn't sung a song as well as I could or should have, but on saying as much, have been told No, it was fine..
I'm my father's daughter; he was one of THE most critical people in the known universe when it came to musical competence, but he was a full time professional musician for most of his life. So I grew up being very much aware of the fact that we can all do better, not because he thought he was the best - he didn't - but because the MUSIC deserved better. (Although there was some music he'd cheerfully tear up for bog paper, but personal taste in music is up to the individual..)
So I might think critical thoughts about the bloke whose pitch goes lurching all round the houses, but I won't usually mention it, unless he does first, and sometimes not even then.