Can you play as a Mii in Sonic and Mario at the Olympic Games?

Yes, you can play as a Mii in Mario and Sonic at the Winter Olympic Games. First of all, you should have already made some Miis in the Mii channel. Then, when you're selecting characters during an event, you can change it from Mario and Sonic characters to Miis by clicking on one of the tabs (I've momentarily forgetten what the tab looks like and where it is). This will allow you to play as any of your Miis. Don't delete your Miis from the Mii Channel or your Mii will be deleted from Mario and Sonic at the Winter Olympic Games.