QR codes appear to be region locked.
Nintendo released the first QR code for Mario Tennis Open in North America, unlocking a Yoshi Costume for your Mii in the game.
To unlock the costume, you have to press up on the D-pad and the Start button at the same time while on the Profile Select screen. Then, take a picture of the QR code on this page and the costume will unlock in every existing profile.
Other regions are also getting QR codes, but they don't seem to work across each region. Other QR codes will unlock seven different colored Yoshis. Check out our guide for more information about the game's characters and unlockables.