Fan art, like Fan Websites, more often than not violate copyright. The fact that they aren't prosecuted is a separate issue all toghther. The Fair Use clause could allow you to copy and use fan-art as a training exercise... it would even allow you to post it online so you could show off your skills. But the second that it comes close to a commercial effort, say with adware or using Mario to advertise your freelance for-pay modeling skills, it will infringe Nintendo copyright.


Note that copyright makes no distinction as to what a "copy" is. This is up to the copyright owner to feel that it's a copy and a lawyer to argue that it is and voila! ...it's a copy.

Stay on the safe side of the law: be original.