On 7-night Disney Magic cruises, the early show in the Walt Disney Theatre goes from 6:30 p.m. until approx. 7:30 p.m. Late dinner goes from 8:30 p.m. until approx. 10:15 p.m. So, you have about one hour between the show and late dinner. (It's different on the one night when there's a movie.)
There's plenty of time to dress between the show and dinner. However, I prefer to dress before the show... and that has nothing to do with wanting to be "dressed up" in the theatre. After spending a day in port wearing shorts and getting hot and sticky, it's nice to take a shower and put on clean clothes for the evening.
As musicgirl noted, there are activites during the hour between the show and dinner. The family cabaret shows in Rockin' Bar D are very popular. There are musical acts in the lounges, with no pressure to buy a drink. Your Navigator will list other events.