So, all birds have a faint bird smell even when they are clean. It isn't strong smelling, though. Personally, I like it.
My own ducks have never smelled badly even when their pool has been frozen over for days. But they do, always, have a faint bird smell.
To give your ducks the best chance at smelling fresh I recommend allowing them to bathe in clean water that you change daily. They don't need anything in this water. They drink the water they swim in. Also give them plenty of substrate to sleep in and spot clean it daily, or as frequently as possible. If they are crowded and sleeping in their poop they might be smelly because of that.
You could also try a product called sweet pdz in their coop. It absorbs amonia and smells. You just sprinkle it on their bedding.