"John Carter Warlord of Mars" would have been fine
As a title for the movie that it's attached to? No, it really wouldn't, because only two words of the five are true. Carter isn't a warlord and the film's plot is driven by Carter's desire to go home to Earth and his "cave of gold," so it's not true until he finds himself and his place in the world (which comes at the climax) that he can be said to be "of Mars." Believe me, I would have wanted nothing more than to see the words "of Mars" in the title, but after seeing the film last night I know that they don't apply.
I actually think that "Under the Moons of Mars" would have been a better title. Though I don't think that Phobos and Deimos actually look the way they're shown in the film.