Don't we need to differentiate between a sing-around environment versus a concert environment.
If you go to a sing-around, you takes what you get. However if you pay to see a concert, you normally have an expectation that the performer(s) is/are capable of entertaining the audience in one way or another.
In all the concerts I have been to or put on, I cannot remember anybody performing badly. Ok maybe they could have performed better, but hey, we all have good and bad days.
What really bugs me, is when people say that a performer(s) is crap just because they don't like their style. I have seen many performers, some I have liked and some I totally disliked because I didn't like their style of music. That doesn't make them crap. I just don't bother to go and see them again. If somebody asks me what I think about a performer(s) who I don't like, I will readily admit to not liking, but then explain their style of music, so that the person can make their own mind up if they fancy seeing them.
Why oh why do people say that somebody is crap, when clearly they are not.
However, if a performer has the habit of going on stage drunk or on drugs or constantly cancel bookings or letting organisers down, then we are talking about a totally different thing.
I would be very very upset, if I turned up to a concert only to find that the artist(s) are badly under the influence. They shouldn't even be on stage. They have a responsibility to their audience. If that is a regular occurrence, organisers should not book them, after all the organiser has a responsibility to their audience as well.

What is disgusting is people publicly pulling other performers apart. That is just not acceptable. Especially as most of these people have a grudge or jealousy or don't like their style. Tell the performer (if they are still alive of course), don't try to ruin their career, just because you don't like them.

How many people haven't gone to a gig, because somebody had told them not to to go because they are crap, when indeed they were not their style.

I'll get me coat