I'd disagree. When I am slumming it at home, I wear the most comfortable pair of trousers I have. If I'd receive a call to have a beer with pals, I'd still change.

I think the same applies in the given example: the in-group knows the guy in the shorts and T-shirt is a nice dude who helps others and is super-professional about quality. The customer, as an outsider, just sees someone who's underdressed.