Sundress is fine.
You might consider bringing something to cover your upper body for those moments when you get overwhelmed by the AC. Also, some nights, it can be cool out on the observation deck. Remember, you’re 150 feet above the ground and it’s windy up there.
Open toed shoes for a woman are fine.
If you wanted to wear (non cut off) shorts and a golf shirt, that would be acceptable too (man or woman). I do all the time.