Depends. Depends. Depends.
Is the book billing itself as a gritty murder mystery? No.Way!
Is the book billing itself as a light-hearted action romp, rom-com, or zany fantasy?? Hail, yay-ahh!
Covers always tell you something about the book and the publisher and what kind of customer they are aimed at. An anime-style cover tells me they're aiming at folks that are familiar with manga and anime tropes and themes. And console gamers.
Just like this upcoming live action movie:
http://www.youtube.com/watch?v=gf-KCPFmc0o
Among all the anime cues in that trailer there is one dead-giveaway of the movie's genre credibility: the computer voice in the trailer is instantly recognizable to anybody exposed to current-generation video games.
Yeah, I'll be there on opening day.
They had me at GUILLERMO DEL TORO + Giant MECHA.
Anyway, covers are *marketing* tools.
Just like blurbs, back-cover text, and titles.
(And movie posters and trailers.)
Done right, they'll help the right customers find the right book; done wrong they'll annoy the browser and cost a sale.