Weird...maybe this is while in "maintenance mode" or something? I guess it doesn't surprise me that some DS games might be able to use Mii's...probably a game on Wii that allows you to use Mii's that is also available on the DS. Mario and Sonic at the Olympics is the only game I can think of that fits that description.
(BTW, you can access the Wii's maintenance mode by holding +, -, and A when you turn it during the safety notice thing.)