Tinker, it is true that most parodies often perpetuate stereotypes, but there is usually a basis for such.   In the case of "A Mighty Wind" there are definite basis for each group and individual in that movie.   Sure, they could have added a sterotyped protest singer, a jug band, and so many others... but that would have bogged down the film.

It is one thing for us not to like a film, butI don't think it is fair for us to tell a director what areas they should cover in THEIR film.   It would be like telling Dylan - maybe you should have added rain and snow instead of having the answer only blowing in the wind.   Maybe John Steinback should have added a few scenes featuring New York City in "Grapes of Wrath".   Sure, there are always things that I would do differently if I was the creator, but I do think it is only fair to judge someone on what they created, not on the art you wish they created.

As for the New Christy Minstrels, I think the issue that most of have with that "genre" is that there is little basis for it in folk traditions. You can see how bluegrass bands developed and the tradition of solo singing is one of our oldest, but where does "neuf-tets" come from???