What I (and others) keep on bringing to your attention is that what the OP was talking about was a venue where she and her friends paid good money to see a headline act but had really crap floor spots inflicted on them, so much so that the newcomers are unlikely ever to bother again.

She did not take them to a singaround or session consisting of amateurs (not that this necessarily will or might indicate lower standards).

This means that the event was, presumably, advertised as professional entertainment. Money changed hands yet new potential supporters, far from being encouraged to return, have been repelled, not by "amateurism" but by "amateurishness". They were insulted, and rightly so and in fact robbed and cheated. Even more importantly, the music which some of us care about has been demeaned and diminished.
Should an organiser have exercised quality control? Emphatically yes.

If your woman who composers songs about her pit town has aspirations to perform them before a paying public, then your organiser should persuade her to learn how to sing her songs in tune first. And not put her on till she can.