As I can see, the influx of japanese users into cloud is simply because so many blogs written in japanese have introduced the mastodon and recommended cloud as the best place to start, especially jpn speaking users.
@Mummy mstdn.jp grew past cloud but didn't exist few days back. Though it seems hard to connect now, so maybe some are choosing to come here instead? Google search does indeed produce several japanese blogs.
@Mummy the "hate" (if you can call it that) that some have towards a local filled with non-Latin characters has been a little disheartening. It's a privilege to have so much Japanese in here because it more honestly reflects the world. English-speakers seem to feel entitled to be part of every conversation, especially online.