Yes the atmosphere would probably be gone but life would not necessarily be gone. If it was there in the first place. I mean, the crater left after the one that killed the dinosaurs was 180 km in diameter and 10 km deep. Not as wide a crater but Much deeper and life obviously survived. Dont underestimate the ability of life, especially micro organisms to survive catastrophe.
Well there are some key differences here. First, life was much more diverse and established here on Earth when the comet or asteroid that took out he dinosaurs hit us. Also, the Earth had deep oceans that protected many species of life from the worst effects of the impact and it's aftermath. Finally there is the fact the Earth is much larger than Mars.
Mars is only about half the size of the Earth, has no oceans, and it's atmosphere is less than 1% as thick as what we have here on Earth. Put all the factors together and the odds of any forms of life surviving the impact or it's aftermath are extremely remote.