Research has shown that in general people will do better in future if they are given praise, so it is always OK to praise a performance if you feel it merits it. On the other hand, if you thought the performance was poor it is usually best simply to say nothing unless the performer specifically asks for comment. Then you need to think carefully about what you say and equally important how you say it.

I think most people who care about their performances know when they have given a substandard performance - though sometimes they may not like to admit it to themselves.

Of course it is always possible that a muted reaction to a particular song may be because the song itself was not particularly liked rather than the performance was particularly poor. There are times when I have given what I felt was a reasonable performance and the reaction was polite applause. I then made a note to myself not to sing that song again, at least not in that location.

On the other hand, there have been times (admittedly, not many) when I have not been particularly happy with my performance of a song only to have someone come along and tell me how much they enjoyed it.