Actually, there is some fact behind this..
I remember reading somewhere that during the creation of the Wii, Mario characters were running around in the Wii Sports.. uh.. sports. Nintendo asked a test play group to test it, and people were choosing to create an avatar instead of use the Mario characters. Nintendo realised that people liked creating their own avatars so much that they made the program deeper and gave it more options..
Thus, the Mii creator was born.