When we went a couple of years ago, we didn’t dress up and I’d go with the 65/35 ratio for adults dressing. We didn’t dress up because MNSSHP was a surprise for my boys (we were at Universal for Wizarding world, but I couldn’t resist popping over to WDW), and costumes would have been a dead giveaway. They didn’t believe we were going until we actually walked through the gates! We took the boys to the Pirate’s league for costumes for the night.
If you want to dress up, you’ll feel right at home! If you don’t want to dress up, You’ll still feel right at home! So, do what you want and have a great time!