I find people tend to obsess too much about clothing. Not everybody wears long skirts or dresses. By all means wear it, I would suggest a shawl or coverup because some of the public areas can be really cool. I feel if you have made an effort of dressing up to show respect to your fellow passengers then whether it's long or short doesn't matter.
Sun Princess (2 times), Mississippi Queen, Dawn Princess, Coral Princess (2 times), Regal Princess, Grand Princess (2 times), Diamond Princess, Maasdam, QM2 (15 times)