I just tried a yoga mat inside my hammock last weekend. It got down to 35F. Over all it was warm enough.
The mat turned out to be too narrow at the shoulders. I added a car window shade under the yoga mat at the shoulders to finish out the night.
The mat bunched up under me and created ridges that became uncomfortable.
Tonight I made a double-bubble reflective pad to replace the yoga mat. It is 10oz lighter and should be warmer.