Mastodon is what you make it and it can be hard to find the community you want. I promise there’s plenty of Japanese art (maybe try pixelfed, friendica, diaspora?) and you can/should unfollow people who just talk about stuff you don’t care about.
This is a very “if Walmart wants to open a supercenter in our tiny little town there’s not much harm in letting them, after all our local mom and pop shops will always be available if we need to kick them out!” – Facebook has done this many times before, by the time the jig is up it’s too late to undo.
Mastodon is what you make it and it can be hard to find the community you want. I promise there’s plenty of Japanese art (maybe try pixelfed, friendica, diaspora?) and you can/should unfollow people who just talk about stuff you don’t care about.