Well, I *think* you are allowed to create a Mario-like kart on yourself if you don't share it with others.* However, I'm pretty sure that Mario as we know him from Super Mario or Mario Kart is copyrighted, so the kart may neither be included in the core game nor in the add-ons.
Besides, STK is a game "featuring Tux and Friends" - and I don't think Tux would be friends with non-free software mascots. So, even if my above assumption about the copyright should be wrong, I would not support including such characters in the add-on section. Please keep in mind though that this is
my personal opinion - if Mario should be free to use (against my expectation) and you find the time to model and texture it with proper licensed artwork, you should be able to include it on the add-on page.
This being said, I wouldn't encourage you creating a kart with a non-free character; however, it's always your decision what to do with your time.

If you are able to create 3D models, creating karts with free mascots will be approved much likelier by the community; and there are even chances (if you are really good) that it may become included in the core game.

* Disclaimer: I am not a lawyer, so this may be wrong, too.
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