It’s a Mastodon type microblogging service but I think I read it’s popular in Japan and South Korea.

There are a lot of forks of Misskey which are more popular in the "West". I recommend you to have a look at Firefish, Iceshrimp, Catodon or Sharkey. Firefish used to gain a lot of traction but there was some personal drama going on with the original maintainer, causing the project to feel unmaintained for a while. Luckily the do have a new maintainer now. Anyway partially due to this problem some folk like the former community manager created a new fork Catodon. Iceshrimp also another direct fork of Firefish. So a bit of trivia from the FOSS Mastodon world. In the you might as well just use Mastodon if you are fine with the UI. But I love that we have so much variety in the FOSS ActivityPub World.