The only people that I really know very well that I would see perform would understand if I told them it wasn't that great. NOne of my friends have ever had a terrible performance, though. But usually they ask what I though about it, I'll tell them what was good (something always is, even if it's just good timing), but I'll tell them what I didn't like in a truthful way. I'll say " I couldn't feel what you were doing in your face", or "you looked kind of sloppy on that part of the movement". I'm not mean to them, but In general, I'm a blatantly truthful person. And my friends know that. They have been performing just as much or more than me, and they are used to criticism. They should understand that you are telling them becuase you care, and you want them to be able to improve. It's to the people that I don't care about or know that I say " the costumes were nice".
ANd I think that if the perofrmance was terrible, then they would know, even if they wouldn't say it out loud!