I think in previous years the term "park casual" was used. It sort of implied that shorts and T-shirts were OK because who brings a sport coat to Disneyland? I'm glad to hear that they have changed to "dressy casual." If you were going to the finest restaurant in your home town, what would you wear?
Napa Rose really is a fine dining experience, and dressing up just a little seems more appropriate for the venue. When my wife and I ate there, she wore a nice dress while I wore a dress shirt and jacket. It certainly made dinner more special for us! At least 85% of our fellow guests had dressed up in some fashion. Those in shorts and T-shirts seemed out of place. Just my opinion . . .