Well, I don't carry many passengers in my Turbulent though I agree with you that it would be unseemly to dress up in a helmet, lifejacket etc... and not offer them the same.
So far my flying gear consists of two nomex long-johns, nomex socks, one nomex long-sleeved top one ex-RAF surplus grow-bag and a bomber jacket. Tuesday was the first day I've flown and not wished I had another few pairs of nomex underwear but I got heat exhaustion running for the train. Often I take a thin fleece to wear over the top of the grow bag on my way to and from the airport. But if I really don't care what people think, then why be so self-conscious?
perhaps less so flying a 172 on a routine day
It used to be argued that driving helmets would prevent more head injuries on an hourly basis, than cycling helmets. Before airbags became ubiquitous this may well have been true. I suspect it would be quite reasonable to wear a helmet in a 172. Perhaps not on an SR22.
I've bought the shell and liner from an RAF Mk IV helmet for not very much money. If it fits comfortably I'll consider cannibalising some old headsets I own to convert it or go for a ready-made one for £220. Otherwise I'm tempted by a Flycom helmet at £500 or so.