Memorial weekend is not a popular weekend at Dumont. The "dune season" at Dumont is pretty much over by most until October. That weekend is usually very hot...like about 100 degrees. Some people still go, but duners usually head to cooler areas like up north; Coral Pink, Little Sahara, etc. I wouldn't expect many people at all for Memorial weekend out at D.
If you go, bring plenty of water when out in the dunes.