Lynn Collins was cast for the part of Dejah Thoris. I have no opinion about her, I haven't seen Wolverine or any other movie with her in it. I assume she must be good because I think Andrew Stanton has good taste. But, I don't know how Hollywood studios operate internally, there are likely a lot of factors other than a director's wishes that goes into the casting process.
But, if I was the casting director there is little doubt who I would cast:
Yes, Olga Kurylenko, the Bond girl from Quantum of Solace. Not only is she devastatingly beautiful, but she has incredible screen presence and comes across as strong and smart. Her accent is "exotic" to American ears. And it is easy to believe John Carter would battle hordes of martians across the face of Barsoom to rescue her.