Not formal at all in Wonder, the seven day cruises in Magic a bit more so. Triton asks for a jacket "if you brought one", interesting choice of words, the other two restaurants just a sports shirt. Women are expected to wear whatever you wear when the guy is wearing a jacket or sports shirt. Most people conform since that is what they packed but there is not enforcement, none whatever. Ballcaps and two day old tee shirts are everywhere. The by reservation restaurant is supposed to be dressy and for the most part was, but a button down dress shirt was often worn in leu of jacket. All I ever saw on women above the table was a blouse or nice tee shirt. Dress nice casual and dress clean and you will be fine for dinner. I went on a bit much on dress in my cruise report on Wonder.
When they say "formal," they mean it. We just returned from the 7-night (WONDERFUL, by the way), and thankfully I checked the postings and learned that had I not had a jkt/tie, I would have felt very out of it. On the formal nights a good # of people wear dinner jkts, even tux's. Next time my 11 yr old & I are going to rent tux's for the fun of it. I know my wife & daughter wishes they had brought at least nicer dresses. It adds to the fun!
We went on the 3 day and the 4 day Disney cruises and they never had a "formal night"...so when we booked the 7 night cruise we never even thought to pack formal clothes......But we were not made to feel out of place by any of the staff....we even ate at Palo's with no sport coat.....wore a shirt with a collar......We are going again (7 day western Caribbean in September) but we'll will bring formal attire this time!