Hi Taiyo,
I've visited mstdn.jp and I saw some disturbing images and no indication that there was any moderation. I think this is the reason the instance has gotten the reputation it has and why so many block it.
Yes, some instances just copy/paste block lists, but mstdn.jp brought it on themselves due to insufficient moderation.
I personally limit mstdn.jp, not block it, because I don't want any posts showing up in timelines or shared to other instances that I'm relayed with. I can't trust the instance. And my instance would risk becoming blocked by others.
It's a shame because being able to fully federate with a well-controlled Japanese instance would be an opportunity to share culture. But the culture it shares is sometimes not something that ought to be shared.
For me, personally, as someone very interested in Japanese culture, I'd love to fully federate with a large Japanese instance, but, sadly, cannot do so with mstdn.jp