Why is Pocahontas in this category? Because she's the chief's daughter, the Native American version of a princess.
Technically her people had no princesses per say but she was presented as one to the English in real life. In the sequel she even refers to herself as a "visiting princess".
In a similar vein, in the Mulan sequel, Mulan is referred to as "The hero of China. A bigger jewel in any crown than any three mere princess" (paraphrased from memory)
While only vaguely implied in the film itself, one must remember the time period and setting at which The Princess And The Frog takes place. Tiana is still going to be plagued by very overt racism, systematic oppression, rape, murder at the hands of the KKK, and pretty much many horrors for many more decades. America wasn't a friendly place at that time for a "woman of her background."
We can always imagine in the Disney universe things overall went better than real life, considering actual racist characters get their comeuppance in the end (Hunchback, Pocahontas), and you could find minorities working in high level fields even back in a time period where it was unusual (Atlantis). We can imagine that while things weren't perfect, in the Disney world Tiana probably had a better ending than in the real world (being a princess with an alligator for a friend would dissuade anyone from attacking her, as well).
A bit meta, but Tiana was also, in early scripts and concept art, essentially a chamber maid of Charlotte, not a restaurant worker. Let that sink in. Disney originally wanted their first Afro-American princess....to be a slave. To make matters worse, her name wasn't the rather beautiful Tiana. It was "Maddy", a classical slave name. Luckily a brief outrage from a leaked early script made Disney back away from all these things.
