>>389
Moomin is finnish. It was made by a Swedo-Finnish person (a finn that speaks swedish, swedish is the second language of Finland because of historical reasons) called Tove Jansson.
Moomin is much more than a cartoon. There were a whole load of books and comics before the cartoon. The literature, especially the earlier stuff, can be a lot different in tone and much more "mature".
There's a moomin theme park in Finland. It's pretty terrifying honestly or at least was when I was a kid. Much like 2D anime characters, moomin aren't supposed to exist in 3D.

Why it's popular in Japan, I can't say. Finns generally know that it's popular there but I don't think many know why exactly.