Can I do Merzouga as a day trip from Marrakech?

No — it is 9–10 hours each way, so the realistic minimum is the classic 3-day, 2-night round trip.

A same-day return would mean 20 hours of driving for minutes of sand. With three days you get the gorges road, a sunset camel ride, a full desert night and sunrise on the dunes — the actual experience people travel for.