I never saw anyone at dinner in PJs on our Merrytime cruise. I did see a lot of families wearing matching PJs at specific Christmas activities or after dinner.
Christmas Eve last year, saw some families in PJs at the shows. It wasn't a distraction but agreed if you are going to do it for pictures after dinner was best.
Pluto's Pajama Party in the kids club kind of requires the kids to wear PJ's...and sometimes the activity is scheduled for 7:30. Going from dinner to it directly...your kids might be in PJ's at dinner to make that timeline work.