We've heard all sorts of explanations about the visuals in StarFox Zero. Earlier during E3, Miyamoto stated that a goal was to have the game running 60fps on GamePad screen and TV, so the visuals had to be pulled back a bit. Platinum was working on making them a bit prettier. Now we're also hearing Miyamoto say that the cartoonish style is done on purpose.
“A lot of games nowadays look so realistic that they all look the same."
Miyamoto didn't want StarFox Zero falling into that, so that's another reason why we have the visuals we do.