The site is Anime & Manga after all.
This wouldn't hurt anything, because I doubt that we'd ever have a manga.SE that wasn't this site, but I'm not certain that it would really help anything?
There's no SEO value, since the redirect is going to be to anime - so making links with it won't really do anything. Anime is just as easy to type as manga, so there's no real shortcut value there (a good example is expats => expatriates).
Is there a more strategic reason to have it? E.g. flyers or other promotional things directed at people purely into manga, where having that specific URL would be like some extra special sauce? We'd need something to justify bugging one of the devs to set this up, beyond the sake of simply having it - did I miss another use case?
Yes, this should happen.
There's already precedent for doing this for & sites that deal with two simultaneous names and topics:
- Linux & Unix: https://unix.stackexchange.com/ and https://unix.stackexchange.com/
- Gardening & Landscaping: https://gardening.stackexchange.com/ and https://gardening.stackexchange.com/
- Science Fiction & Fantasy: https://scifi.stackexchange.com/ and https://scifi.stackexchange.com/
- Movies & TV just received a request for a similar redirect (movies.se and tv.se) which is my impetus for leaving this response.
Anime & Manga Stack Exchange is in this same family, and should similarly have both https://anime.stackexchange.com/ and http://manga.stackexchange.com/, so that a person visiting this site by either name finds it.