Compared to other cruises that I've been on, the fashion leans towards casual on the Disney ships.
You will definitely not feel as if you've embarked on Fashion Week. Casual truly is the norm aboard the Disney Magic. I also think that given the strict airline luggage policies that many families are packing lighter. Thus, Dad may be leaving his sports jacket at home in favor of just wearing a nice shirt and tie.
The only
guidelines for the dining rooms are no shorts, tank tops or swimwear. So, on "Cruise Casual" nights you'll see women and girls in sun dresses, capris or dress pants/jeans. On casual nights, there were many passengers wearing denim jeans, including the Jr. Cruisers.
On Formal night, I saw many men wearing suits and only a few donning tuxes. There were also a lot of guys just wearing a dress shirt and tie with no jacket. Women and girls were mostly in dresses. I saw very few floor length dresses or "prom" type attire. Women mostly wore cocktail, party dresses. Teen girls wore party dresses too.
Semi-formal night was mostly dresses and/or skirts for the ladies. Men wore suits or an outfit with a jacket. Girls wore casual dresses, including sun dresses. Some also wore cute skirt outfits.
One way to get a clearer picture of the fashion aboard the Disney Magic is to google "Disney cruise formal night" and look at the images tab at photos. You'll get a glimpse of the wide-range of fashion that is unveiled on Formal Night.
Also note that you may want to bring a lightweight sweater, wrap, jacket or shawl as the dining rooms and theatre tend to be chilly.
You won't feel out of place if you do not "glam it up" in formal length gowns but it is fun to get the family dressed up for dinner. Take advantage of everyone looking their best and have family portraits taken!