Then you'd need to add Manhwa, too. It almost seems like book categories need subcategories, i.e. Manga and Manhwa are subcategories of Graphic Novel or more generally comics / sequential art. There's also a bunch of similar categories looking at the Wikipedia page on Manga.

I think the most helpful aspect of having something marked as Manga is being able to determine the page order as being right to left instead of left to right. It's really helpful to be able to get that kind of info. But, being able to distinguish manga from western graphic novels... yeah, that'd be good.