The truth is: It probably did hurt them. If they had just worn denims and knuckled down to it in working man's fashion a là Quo they might have had a few less Top of the Pops appearances and hence hit singles, but more appealed to the serious rock crowd and album buying public plus fetched some better opening slots on large scale tours in markets they did not yet rule with their singles output. And I think it would have benefitted their US career too, never a country to embrace glam except in that mutated larger-than-life version of Alice Cooper/Kiss. You could look unremarkable and still crack the US market as Foghat have amply proven.