Seems to be applied to almost everything after checking the results of the tag. Is it common in Japan SNS?

Yes, it is.
It is a cowardly way for sub-tweeters to mock people behind their back. Rather than go to that person and directly discuss about the issue, they use this hashtag.

Ah I didn't know that. At least it's a good way to find the slime balls.

I guess I will have to figure out Mastodon filtering...

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Fedibird

様々な目的に使える、日本の汎用マストドンサーバーです。安定した利用環境と、多数の独自機能を提供しています。