This is such a cool concept! If you're still editing, i agree that Mario shouldn't be taller than Luigi.
Also, on the back, you could play with the pun a little more. It would look great to have several pipes protruding out of either side of the shirt, layered horizontally instead of vertically. Then it would look like one of the classic obstacle courses, because right now it would kind of look like a tail on the wearer.
About my design