There are many paid performers who have been singing and playing for years who are dire and unlistenable to, but that is why beer tents were invented, to get away from them. But these are the same artists who are revered and followed by others, so once again, it is all a matter of personal taste.
I am a lot more intolerant of booked artists who have not bothered to entertain their audience in a balanced and creative manner, than a few off key, or inaudible souls who have a go for nowt!