Youth fashion has always loved a bit of irony: "It's cool because it's not cool." Even if you don't understand it, it does seem to work as an effective way the kids to tell parents that are not going to play by their rules. In generational terms, irony-laced fashion tends to spawn from two key factors: when youth want to show a middle finger to the previous generation (how else do the the kids of a Comme des Garcons aficionado rebel?) or when they are firmly priced out of the fashion market leaving only older and affluent besuited workers able to buy it.
In Japan, add to the mix the inbound tourists who throw cash at stores selling gear aimed at young millennials who can't afford it, and you have the perfect conditions for an ironic revolution. For the cash-strapped youth, the many vintage shops of Tokyo's Koenji, Shimokitazawa areas, despite being routinely picked over, are mercifully still on hand for the Hawaiian shirt or GI belt that kids used to mock dads for wearing.
Still, fashion being the hungry beast it is, the irony trend is ripe for harvesting — and it has led to a curious mix of people wearing clothes ironically and people wearing ironic high fashion seriously.