There's a contest going on (check Miitomo announcements to see the details) where they are doing this T-shirt fest starting with Mario characters (or it may just be? Idk), and to start it off, they are having people vote which of 4 designs to have a free giveaway for. Then, others will be available through Miitomo Drop stages and My Nintendo rewards. For the contest, there are 4 designs to vote from, and the poll ends tonight. There's Bowser, Goomba, Boo, and Blooper designs, and the last time I checked, Boo was in the lead.