"Just take it from me: I'm an OK singer and an uncalled-for criticism really spoiled my evening."
Then I've misunderstood you C.H. and I apologies and totally agree with you.
I don't think I encountered many unjustified and unsolicited criticisms, but I'm sure they exist - they certainly do in other fields of art.
But the general point still stands, sincere criticism is too often mistaken for condemnation and as a whole, the British folk scene as a whole seems to have undergone a course of injections in order to ward off any form of critical discussion on performance.
Like your choice of name BTW - one of my favourite songs.
Will
In the long run, your most immediate and accessible critic is your audience, whether they are good singers or not - they are the ones you have to move if you are going to sing in public.   
Jim Carroll