AIUI, Kobo devices have a more advanced rendering engine if you name the file with .kepub.epub. (I think it's based on ePub 3?) Not sure if it would fix the problem here. But I personally run ePubs through kepubify (https://pgaskin.net/kepubify/try/) before transferring them to my Kobo.
The graphics would be called chibis, IMO (or デフォルメ if you wanna be fancy) and IDK about developers, but perhaps weaboos/weebs would be the general term
At least they appear to be partnering with Kobo "later this year" [1]. I've been a big fan of Kobo's devices so this is a nice plus. (I just wish they could figure out some way to get Kindle exclusives, but well that's a contradiction in terms, so...)