I can't believe I made an account specifically to answer this one point, but as a staunch defender of this movie, I have to point out that the reason Splinter has an Asian accent is because -- as a rat -- he was from Japan.
Also, let's be nice about Keno. Ernie Reyes Jr. is going through a lot right now, and for a LOT of Filipino/Filipino-Americans (like myself), Keno was the shit. Next to Rufio, naturally.