No experience of doing this but plenty of experience of thermarest type mats. I doubt it will work at all. They are generally a little slippery, at least on one side (even the soft faced ones), and it'll quickly slide down and end up all creased and heaped up under your feet (certainly no where near your head).

I'd think more about some sort of pillow than mat if you're trying to isolate your ears / head from vibrations.