Order a doll of your Mii Wii
"We are specializing in custom hand made sculptures. We have a team of artists, create sculpture base on photo you provided. ... You can even send us your photo of Mii and your Mii number."
They're expensive, at $75, but they're hand-made by a living, breathing artist. That's a low price to pay to transform an empty, soulless simulacrum of the human form into a cake topper!
Product Page [Mii Sculptures]

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s/Mii Wii/Wii Mii/
Thought I'd share another (and IMO better) Mii cake topper (I think I saw it on the mother Boing).
http://paulpapedesigns.com/Me-cake.html
Ugh! They're creepy enough on-screen. Way worse in physical form.