In answer to your original question which was "would we look out of place if we changed into shorts for the shows?". We are young(ish) cruisers, and were on the Westerdam at the end of last year, and, "yes", on formal nights, the one or two people that changed into casual clothes after dinner, did stand out. Before the cruise we thought we might change after dinner too, but once we'd made the effort to get "dressed to the nines" for dinner, we thoroughly enjoyed the experience, as we never get the chance to really dress up at any other time, and having been in the situation would have felt very uncomfortable dressed down, among everyone else.
Have to say that much depends on the ship you're sailing and the itinerary - the Zuiderdam, from the very start of her career has been just a bit different from HAL' s other ships..She appeals to 'a younger' more casual/sloppy crowd. Not all HAL ships are the same, praise be.
There's another point about changing after dinner that no one has mentioned - the temperature in some of the lounges is such that you're downright cold if you change into shorts - the movie theatre is a good example - I always take one the the large towels with me - it's the only way to keep warm!
Actually the larger the ships the more they resembles land resorts so, to each his own. We no longer take 5 ports in 7 days itineraries - in fact we no long take 7days vacations.
Sloppily dressed cruisers will not ruin my cruise experience but I do expect my dinner companions to dress decently - by that I mean at least a nice clean shirt and decent slacks. I would ask to be moved if I had to sit beside or opposite someone displaying a hairy chest or bare below the navel midriff. There's casual and there's casual!!!!!!!!!!