From what I know about rosehip oil, I do not think that it will improve acne scars. Very few topical treatments improve acne scars. I recommend you consult with a board-certified dermatologist experienced in treating acne scars for evaluation of your scars and for a discussion of treatment options. In the meantime, if you want to try the rosehip oil, I do not think there will be any harm done other than the chance that you may have a skin reaction to the rosehip oil.
Web reference: http://www.barnettdermatology.com/treatments.php?id=24