Even though it is already possible to edit your Miis on the Wii, there's a new piece of equipment that is available for you to import them onto to your PC, edit them, and then re-import them back on to your Wii.
Some people want to completely customize their Miis, which is understandable, but the point of using the Mii software on the Nintendo is to make them look Nintendo. That's at least my favorite part, I already think of myself in real life as a Nintendo character, so it does as a bit of a shock in the morning when I wake up, look in the mirror and find out that I'm not.