マミーちゃんだよ！✅ 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.
Juha Makkonen@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.