True Left

"So would opposing jeans necessarily make one Western?"

Not necessarily. But anyone who opposes jeans MORE THAN they oppose anything within Western dress code (informal and above) is Western, since in effect they imply that Western is superior to Counterculture.

"Shouldn't non-Westerners also oppose jeans since they are after all a Western invention?"

You may personally prefer some non-Western types of trousers over jeans, but you should still give credit to pro-jeans fashion movements for the significant part they played in chipping away at Western dress code during the Counterculture era:

Distressed denim emerged from the cultural punk movement in the 1970s. Early punks tore apart consumer goods as an expression of their anger towards society.[52] Johnny Rotten of the Sex Pistols manifested the British punk ideology, which was fighting against the status quo. Denim became a key target of this politically fueled deconstruction, with both men and women donning torn pants and jackets, accessorized with safety pins and slogans. The trend became popular again in the 1990s with the emergence of grunge fashion. If punk was "anti-fashion", grunge was "non-fashion". The grunge youth wore loose-fitting ripped jeans, flannel shirts or woolen Pendletons layered over T-shirts. Their anti-conformist approach to fashion led to the popularization of the casual chic look, a trend which continued into the 2000s.