I went through CMN a couple of times over the Christmas period. It's not a nightmare, just not massively convenient either. Just have your passport and onward itinerary handy and pack a bit of patience. 3 hours should be plenty to clear immigration, claim your bag and recheck it. In some ways it's more pleasant as the international-domestic transfer involves walking a long corridor and then waiting in what feels like a warehouse for your flight. Going landslide at least gives you the chance to walk a bit, and relax in one of the cafes.
If you are really worried about luggage then the train is an option. There is an hourly service from the airport to Marrakech (changing at Casablanca ) which takes around 4 hours. You can buy tickets online, with a reserved seat and it's cheap, like $30 cheap in First class.