I hate the word "kiddy" because it's thrown around so casually like "liberal", "fanboy" and "gimmick". The only games that are really "kiddy" are the ones targeted specifically for little kids (such as Dora the Explorer: The Game). Mario is not kiddy, he just appeals to a wide audience. And if you think SSB is kiddy, then there are some issues.
Nevertheless, the Wii is getting "mature" games: Prime 3, Sadness, Disaster, HAMMER, Manhunt 2, No More Heroes, and more. People stupidly assume that because Wii Sports and Wii Play sell so well, the Wii will get nothing but kiddy games and mini-games.