For those new to hammocks:
Note that it is something of a trick to use a thermarest pad inside a hammock. It can be quite humorous to watch the uninitiated struggle through their first night until they get the hang of it. If you simply throw in a pad with a sleeping bag on top, the pad will in short order "squirt" around and find its way to lay on top of the sleeping bag. Not an effective situation.
But there are several solutions. One is to use a light overbag. Put sleeping bag and pad both inside the overbag and your are done. For years I used an old sheet sewn to make a bag, and it worked very well. A second solution is to use a sleeping bag, such as those made by Big Agnes, that has a pocket slot in the bottom that you can slide a pad in. That's what I do now. Or you can fasten velcro to the inside of the hammock and the bottom of the pad.
I have a hammock with mosquito netting. When it's cool enough to need an under pad, I hang the hammock upside down and zip the pad inside. It does have some minimal shifting but nothing like sliding up on top of me. When I need the netting for bugs, it's generally warm enough that I don't need the under insulation.