*Spoiler alert, I saw the movie a couple weeks ago on ABC Family.
Tiana is a princess. She ends up marrying Prince Naveen at the movie's ending, thereby becoming a princess, kissing him and braking the curse wraught by Dr. Facilier.
Funny thing is, after seeing the movie, I think the creative staff behind Princess and The Frog must have thought along the lines of:
"Hey, you know what would be really funny? Okay, so we've got the story where the girl kisses the frog turning him back into a prince. Remember that old Sesame Street News Flash sketch? The one where a prince rejects all the other suitors because Kermit's there covering the event as a reporter, and she heard the same story... And then after she kisses him, instead of the frog turning into a prince, the princess turns into a frog. Yeah, let's use that for our movie!"
As for Mulan, I've got nothing. But at least she was given a prominent role as friend and bodyguard to both Aurora and Prince Phillip in Once Upon a Time.