Is it manga as in 漫画? If so why is it not 日本昔話まんが but まんが日本昔話?
As @naruto cited in his comment, it is a title of an anime series (not manga medium-wise, though).
If so why is it not 日本昔話まんが but まんが日本昔話?
日本昔話まんが would sound like "a Japanese fairy tale comic", and it is a valid phrasing as such. But here, the name まんが日本昔話 is almost surely read in two parts as まんが・日本昔話, and similar to what you might say in Engilsh Japanese Fairy Tales: The Comic (of course, again, it's actually an anime).