Dr.Mario appears as an unlockable alternate costume for Mario in Mario Tennis: Heroes Vs Villains. If Mario completes his Doctor Cup or scans the Dr.Mario Amiibo the costume will be unlocked and you will be able to use it in almost every mode. Invincible Tanooki Dr. Mario also appears for the first time. If Mario uses his Special Shot while the Dr. Mario Costume is in use the Invincible Tanooki outfit will have Dr. Mario’s head piece.
An amiibo figure of Dr. Mario exists! For more info about this amiibo, see this page.