OP, I posted just about this same question last Thursday! DH and I went to NYC for the weekend last weekend, also seeing Mary Poppins. It was fantastic! I wore black dress pants and a nice top, DH wore business casual type clothes. I would say most people were about like that, with some being a lot more dressed up and some in jeans. The next day we saw Phantom of the Opera in the afternoon. We weren't planning on seeing it, just decided to get tickets in Times Square. We were in jeans and I was even wearing sneakers, I dressed for comfort for a day of walking. I was afraid we would look very underdressed (more because of the shoes than the jeans) but many people were dressed the same. I saw very few who were dressed up at all. Later that night we saw South Pacific, once again, on a whim, not planning to see it, so we were wearing the same thing we were wearing earlier. I wasn't too worried as most people had been so casual earlier and even at the evening show the night before, but once there, I felt VERY underdressed in my jeans and sneakers! Oh well, we had fun, the show was great, and I'll never see those people again!
Have a wonderful time! Mary Poppins was amazing!