...I suppose as one of my grandfathers always wore a cap indoors (and a homburg outdoors), being follicly challenged, I'm in no position to carp!
I think US bands have always dressed fairly formally. I remember being surprised to see Peter Paul and Mary in tux & evening gown while UK folkies at the time (1960s)were in jeans and checked shirts, and a blues package where Big Joe Turner and Willie Dixon between them wore natty suiting that must have used up the entire silk production of Asia for a year. However, on the same bill, Bukka White and Sonny Terry wore dungarees. The Clancys' Aran sweaters and the Spinners' & Corries' blousons were a sort of uniform too, all the trad jazz revival bands tended to wear unifoms, even if only matching waistcoats (vests). Nowadays blues bands tend to wear varities of black though last nigh Robin BiBi referred to his bass player's T-shirt ( a blue check affair)as a washing-up cloth.
But I'm rambling again, where's my medication....?

RtS
(wearing medium blue chinos and a black on white "Prewar blues guitar" T-shirt)