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!!!!!!!!!!