In my experience, yes. People without Park Hopper tickets want to get bang for their buck and don't want to be kicked out at 6pm. People also want to come on days where there is nighttime entertainment. Non-party days are packed in the evenings with people wanting to see fireworks over the castle. And most of those people were probably there most of the day. So when there's a party, the crowds are lower than the surrounding days. Maybe not low in general, but during that time of year, the absolute best option for touring MK with less people.
I accomplished 16 attractions by lunch time without rushing around, plus a couple snacks and photo stops. I rode 7DMT at 9:30am with a 25 minute wait time. Worth noting that there was EMH at 7am and the park opened at 8am. More than 2.5 hours after people were let in the park, the line for the most popular ride was only 25 minutes. Party days rock! (Until the secret gets out. Shh!)