It's more than possible. Except for the lack of harddrive space, the wii has superior specs to the xbox, which has a pretty decent port of mame on it. Of course the xbox is practically a pc, which made it easy.
Here's the catch though.... the wii has mac-like architecture and the number of developers that are familiar enough with mame AND are familiar enough with a mac-like system AND would be willing to devote a lot of time tweaking mame to get it to run on the wii is extremely few. Like 1 or 2 people few or LESS. It'll happen eventually, all modern consoles get a fairly decent mame port, just don't hold your breath.