Hmm I dunno. I think it depends on the banada/headsweat thingy.
My hair is only abuot 0.5" long, and I use a cotton bandana folded in a triangle as well. Assuming I don't let it bunch up anywhere (which would affect the fit of the helmet), I have trouble seeing how it would hurt the performance of the helmet during a crash. The extra thickness of a cotton bandana would be simliar to just having longer hair. I dunno.