I think that dressing for dinner, whether it's formal/elegant night or not is entirely up to each person and their mood that night.
On my 1st cruise with my then 14-yr old son, we just wore shorts and t's - of course we didn't eat in the dining room, but we had a 'formal night" photo taken anyway...
But on this month's cruise with my mom' I'm wearing a little black dress and heels. it will be fun to play dress-up, since I have rare occasion to do so. I never care about who else vis wearing what.
My vacation is all about me, not anyone else!!