I used to sleep in a net hammock like they sell at the BX/PX on military bases...the cords were about the size of 550 cord inner core...it would certainly be much lighter, and you'd use about 1/7th of the 550 cord by using the cores. And it would still be strong enough.
The only real problem I had with it was that since the cord was so thin, it would snag on anything I was wearing, like the corner of BDU pockets. And god-forbid it snagged a button...then I was stuck there until I could unbutton myself! But I got several years use out of it.
I think making an entire hammock out of 550 cord would be heavy and overkill...and kind of a waste of of 550 cord b/c you don't need anything nearly that strong. But if you already have the 550 cord then you might as well use it for something!