Hammocks compress any insulation you are wearing against your back, making it worthless. The hammock presses up against you tight and your back will often see more cooling then if you were standing up.
With out underinsulation, I get too cold for comfort around 70deg, and I sleep kinda warm.
Nothing is comfortable if its truely hot... And if your a sweaty mess pressing sil-nylon against yourself wont be great... But that aside you will generally be cooler in a hammock then standing.
Edit: bug nets can get stuffy though, as mensioned above.