We were there last year during a grad night. I don't think the park was any less crowded because of it, but it wasn't more crowded either (since they bus them in late).
But right there is no MM on those days, and they close at 9pm. It didn't effect us much, we just stayed longer during the day (rather then the usual time we'd go back for a break) and then spent the evening swimming and relaxing.
Looks like we'll have another one when were there on June 5th. Maybe we'll do Universal that day.