Tried the yoga mat thing Friday night with both the kiddo and myself.
Kiddo had a double layer tablecloth hammock and it worked great between the layers. It didn't move much regardless of her squirming and she stayed toasty warm using her sleeping bag and the mat.
I was not so lucky. I have a single layer hammock and was directly on top of the mat. Just trying to get into the hammock it wadded up and shifted around. I was miserably cold the first night.
Needless to say I walked back to the house the next day and frankensteined myself an 3/4 underquilt from an old sleeping bag. MUCH warmer and easier to adjust.
Unless you have a double layer or plan to use it only under your feet, I suggest you skip the yoga mat and use your money elsewhere.