Since I first got involved in running folk clubs, organising concerts, festivals, etc., I have never once considered "looks" as a criterion in booking a performer.

If I had, I would probably have been forced to excluded some of the icons of folk/traditional music.

On the other hand, of course, I've seen some traditional musicians turn up in a suit and tie (Seamus Ennis, comes to mind.

I think one of the things that drew people to folk clubs (particularly in the 60s) was this sense of an underground/alternative culture which was at odds with the conventional music venues. You didn't need a mohair suit or bee-hive hair-do to sing a song.

I recall one singer of traditional songs (he was a full-time professional) laughingly tell me that he claimed expenses for "stage clothes" on his tax returns. Never saw him in anything other than jeans and a t-shirt.