My wife and I have sailed Celebrity a number of times. We enjoy getting a bit spiffied up. Last time we cruised we took my parents, my dad is not a "lets dress up" guy. He played along for the first chic night, but the second time he wore a Snoopy Hawaiian shirt with jeans. He got so many compliments that night and not one criticism. I imagine there are pretty a few stuffed shirts who complained under their breath, but he enjoyed himself, we enjoyed ourselves, he brought joy to numerous people who loved seeing that shirt. Ultimately if anyone had a problem that says more about them than anyone else.
My advice - do you. Don't worry about others. I've never seen a single person turned away from MDR on Celebrity, and I've seen some interesting outfits you'd expect to see on Carnival not Celebrity.
Just my .02.