It's true: Barsoom doesn't have to look like a Frank Frazetta painting; it could look like the cover of a book published in 1912.
This is in fact one of my favorite illustrations of Dejah Thoris. I'd certainly be happy if Pixar decided to go with retro esthetics in their upcoming movie. John Carter is, after all, supposed to be a Civil War veteran, so it'd only make sense if the whole thing had a pre-Great War look to it.
Yes, yes, I know: there's about as much chance of that as Flash Gordon ever again being portrayed as a professional polo player, but a guy can dream, right?