I don't think she's like Mitchell, Bush, or anybody else. I think she's been cursed with the decision to use Sakamoto's members as a backing band one-too-many times and is more overshadowed by that than by English-language comparisons. Regardless, her work is very good, very consistent, and gohan ga dekitayo is a favorite album of mine.
Personally I draw lots if similarities with Kate Bush tbh. She's like the Japanese Kate Bush, vocally not really that much (although in some tracks very much so) but musically she's very alike. Not afraid to do her own thing en experiment just like Bush. Perhaps it's also not a coincidence Peter Gabriel got to collaborate with both (but then again Gabriel collaborate with a shit ton of diverse artists). Yes I know it's a 7 year old post, I don't care.
I hate playing the "they're just like a Japanese version of _______!" game with artists, but I'd say Yano shares much more stylistic territory with Joni Mitchell and especially Rickie Lee Jones than Kate Bush.
Stylistically, yes.. Someone overheard me listening to Wuthering Heights and recommended me Yano Akiko, calling her "the Japanese Kate Bush". I don't know to what extent, but I can kind of see what she meant.. though it's different, since Kate Bush was only really big on the piano songs for her two or three first albums.. while with Akiko that seems to have been her base for a long period of time. Kate is really versatile with her use of instruments.
But then, I've only heard two of Akiko's albums: Japanese Girl and Super Folk Song, so I can't really judge her.