It's already been mentioned, but Mylar SBs don't stop conductive heat loss. Conductively, they are probably worse than just a piece of plastic because the shiny is aluminum. They are radiant barriers that need space between it and you to work.

That said, it is a great moisture barrier, and if you put leaves or some other filler on top before you sit/lay down you'll get the radiant effect working. It's also not any heavier or bulkier than the same sized piece of any other type of plastic, it's great for reflecting the heat of a fire, and pretty easy to spot from the air. . . . so why not?

If you just want a thin, compact piece of plastic for moisture control, plastic drop cloths will be found in the paint section of any large home improvement store. They'll have sizes down to about 5' X 8' and they are folded flat and very compact.