According to the Disney criteria Mulan is an official Disney princess because she meets the requirements. Tinkerbelle isn't a human so she doesn't make the cut.
Not saying she's better or worse (actually she's possibly the most badass) just that she never married a prince and wasn't royalty to begin with. She married a general (back then that was basically marrying royalty but still not a king or prince so don't count)
She's a freaking warrior that saved fucking China