I found that you saw all kinds of dress at dinner. I would say that there were a lot of jeans, but often with a collared shirt for men and a nicer top for women. Some ladies seemed to have a new dress every night. My sister and I either wore jeans or jeans skirts with ballerina flats, a nicer top and a cardigan for dinner and felt not out of place. We had dresses with us for formal night, but not ball gowns. But there were people in Disney sweatshirts as well as people in suits and fancy cocktail dresses. Quite a few adult ladies dressed up as Elsa for formal night and/or Frozen night. Some more like a real costume, some more in the "Disneybounding" style where you use the same colors that resemble the character costume. Since the dress code that Disney publishes is so easy now (just no tank top and no swim wear I think), I would really not worry about bringing too many dinner clothes.
We had dinner at Cabanas the evening that we sailed away from Geiranger. We are not keen on lobster and it was Captain's Gala menu night. We got there at 6:45pm (they opened at 6:30pm) and asked for a window table and had the most amazing view sailing through the fjords! I was amazed that there were not more people! I had a fantastic steak there, so it was great food wise, too.