I think that perhaps fwjc overstates it a bit.
At smaller clubs and grass strips a green growbag is neither unusual nor frowned upon - it's just a personal choice.
At a certain type of high-cost-medium-quality GA club people get arsey about all sorts of things that really don't matter. They can however often be places to do ad-hoc bits of renting or training, so you just live with it.
For example, I flew as a sole or part owner at Popham and another nearby smaller strip for a decade or so, where my growbag wearing habits never got even mentioned. Flying at a commercial school elsewhere in similar aeroplanes, but for my CPL, anything that didn't resemble either a baby airline pilot uniform, got regarded with mild ridicule. In the latter case, I told them I didn't care and why I wore it, and they heard me out and accepted it.
Either way, don't be pretentious, don't mind explaining your reasons politely, and people will get over it.
But GQ - do get out and see some club environment flying. At the very least go to the LAA rally in September.
G