Upside Down Face
One of the more popular Mii cheats is the upside down face, which turns the face of your Mii character upside down. Select "Customize Your Mii" from the Main Menu and change the head shape to oval. Choose to put the eyes at the bottom of the face and make the eyes symmetrical. Then choose no hair and select a round nose. Select thin eyebrows and lay them at the top of the head, then tilt them close together and shrink them down, which makes your smile. Pick a flat mouth, put that on the bottom of the head, under the eyes and it will look like a unibrow. Then select a beard and hair color, add it on and this will make you Mii look like it has an upside down face, with hair growing out of the bottom.
Alien Face
You can also use a Mii trick and make your character look like an alien. Start by picking the dot eyes, which look like small dots, and adding them to the face. Using the mole feature, place a mole right between the eyes. You may need to shrink the mole down until it's the same size as the eyes. Choose the straight eyebrows and choose to make them black and the largest size possible. Add them to the top of the head for the antennas. Opt for no hair on your character, then use the smallest nose, which has two small nostrils. Pick any mouth that you like, and once you're done, your Mii will look like an alien.
No Face Mii
Make a Mii that doesn't have a face by putting the long winter hat on your character. Select the handlebar mustache, or any long mustache, and add it to the face. Select eyebrows and eyes, but hide those behind the hat. Pick the smallest nose and mouth you can, but shrink them down further and hide them inside the mustache. You have to pick facial features for your character, but this helps you hide them and make your Mii look like it doesn't have a face.