I think the most you get to character customization is the Mii mask. If you want to know how to make your face I heard that when Rover ask the questions the top answers are usually more "bright" thus giving you a more "bright" outlook, big eyes, smile, rosey cheeks. And the bottom answers are the opposite. And then there is everything in between so you have to determine what gets what.
Miis just take away from the experience of playing a game. If they were used as avatars or icons in games I'd be happy but actually being in there is just annoying. Like in Mario Kart Wii, I hated them.
Although it is useful for Halloween costumes and stuff... makes things more intresting.