You're thinking like an old person here (depth of knowledge, primacy of the lived-experience, familiarity through reading and more conceptually rich mediums). In a web-centric visual culture you can consume images very quickly and harness them in any way you want to, not caring all that much about their insertion in a wider context. You're consuming very vaguely defined signs and playing with them, something that we sometimes "accuse" the Japanese of doing due to a lack of cultural familiarity.
That's an interesting point. but the disassociation between clothes and social rules/ culture predates the internet. Even predates Japanese menswear culture, where things were presented in a kind of sterile, "adopt what you'd like" catalog form. All of 20th century clothing history is a slow march to "liberalizing" clothes -- kind of in-step with general liberal movements (in the European sense of the word). Basically allow people to do whatever they'd like, including dressing however they want.
And to some degree, the original meaning of so many clothes are so far back in cultural memory that they're basically pointless. The reference point for a double-rider jacket is no longer Marlon Brando and The Wild Ones; it's Kanye West. So it's more "cool musician" than "biker." Newer generations imbue clothes with their own meanings, which take them further and further away from their original, cultural contexts.
The internet may have sped all this up. I don't think any of this "wear a Megadeath tee just because you think it's cool, but don't listen to the band" would have been possible when I was in high school during the '90s, but I assume we would eventually gotten to this point with or without the internet. A 19 year old today associates wearing metal tees with Kanye West, who at this point is basically as much as fashion icon as he is a musician, not some grungy metal head with long hair. Again, it's that "clothes take new meaning over time" point.
Put it this way: you can wear a Megadeath tee right now without irony because Kanye said it's cool. You can't wear a Garth Brooks tee without irony (or without being a fan) because it still has cultural meaning. Neither can you wear a dress, cowboy hat, or any other socially loaded piece of clothing. A lot of this is a combination of liberalizing clothes (to some degree) and basically pop-culture driven trends (which are very specific in scope).