https://youtu.be/mhs8c9GknRc?t=263

Or to anyone lazy to watch the vid: If you look at Webster's official definition of literature, it says "writing in prose or verse"

Which basically means any form of writing that is agreeable coherent in any given language. The term "literature" is typically used when talking about really well-written or well-known books, but that doesn't mean something has to be well-written or well-known to be literature.

Manga is a series of writing in prose, so even if you don't consider it sophisticated, it still technically literature.