Is Muumin hentai?
No
Rated G (All Ages) on MyAnimeList and not tagged as Hentai.

TV · 1969 · 65 episodes · G - All Ages
Adventure, Fantasy, Slice of Life
Very loosely based upon the Moomin books by Finnish author Tove Jansson. Jansson, however, never really approved of the series due to dramatic changes in plot, atmosphere, character personality, appearance and names etc. The series was directed to adults rather than children. (Source: Wikipedia)
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