I hammock camp all the time. I use an underquilt since I thrash and toss and turn when I sleep and the underquilt generally stays in place. Pads do not.

I used to use a mummy bag as a top quilt (not zipped up) but now have a dedicated top quilt. Either one is good.

Early in my hammocking experimentations, I took an old heavy Gerry mummy bag, de-hooded, de-zippered it (zipper was broken anyway), and turned it into a makeshift underquilt. Worked OK but the weight of the thing pulled it away from my butt, leading to drafts.

My second, and sucessful attempt at an underquilt, was to take a Ray Jardine style homemade polypro quilt, gather the ends up, and attach it to the hammock suspension. Worked well. I still use it for lounging on the stand in the back yard when it is not summer.

I switched to a down underquilt (3/4 length) to save on space in my pack (most of my hammocking is on foot, for the single over night tour I completed on my recumbent, see this video.)

Whatever you use for bottom insulation either has to be put in a place where it cannot compress (underquilt) or be made of something that will still insulate when squashed.