Not that hard to do. I own three Fawful shirts and a Dimentio shirt. Of course, I got two of them from a guy who made custom shirts and the others from a custom shirt website. One would think the Baby Mario Bros. would be important enough to already be on an official shirt though. Maybe a Yoshi's Island shirt?
Anyway, if the marketing plan is changing hopefully the Mario RPGs will be advertised more, as they're currently obscure and hideously underrated for games that have much more content than the main Marios for the most part.