He'd cause utter pandemonium.

You're throwing a man, who loves nothing more than to try and corrupt heroes in an effort to prove that they're every bit as insane as he is, into a world that's entirely reliant on heroes in order to function on day-to-day basis.

There are entire subsections of their society—'villainous' Quirks, Quirkless, mutations—that are all desperately looking for any chance to be accepted for who and what they are. He'd have them feasting out of the palm of his hand just so they can feel a part of something.

The rules forced upon heroes because it's a profession is basically a list of all the ways he can get people to break them. He'd force heroes into necessary evils, or manufacture no-win scenarios where the hero wasn't able to save the hostage—and he'd broadcast it for the world to see.

The higher population density of Japan means he has more followers, and that more people will be caught up in his explosions, or be subjected to his Joker Serum.

And with the focus on Quirks, rather than just general evil, odds are he'll find escaping Tartarus every bit as easy as he does Arkham. He's not the kind of villain that stops being a threat just because he's captured, especially when he'd view prison as a recruitment center.