There are two different 'big production shows' in the main show lounge on every Carnival cruise. (Dont miss these - very entertaining and 'Vegas-y'.)
Carnival has a good system for making sure all passengers have seating. The 'early' diners go to the first show, around 8:30pm. The 'late' diners go to the second show, around 10:30pm. I usually arrive about 20 minutes before show time, and have never had a problem finding a seat. The main show lounge seating is usually on two levels.
For the other shows; i.e., comedians, magicians, etc., which are usually held in one of the smaller lounges, it's best to get there as early as possible, as the seats are taken quickly. I rarely go to these shows anyway. I agree that some of these should be held in the larger lounge.