Verified answer

Mylar, No. metalized mylar, yes.

No. Mylar is a clear plastic that does nothing to reflect radio waves. The aluminum filled balloon material you see in stores and as plastic signs and mirrors is added filler and whether it would reflect radio would depend on exact content and density. But beyond that, you would have to supply a supporting structure to give it shape to bring the waves to a common point/line for the actual electrical work.

And it crumples in storage.

Any plastic could not capture any RF signal. Antenna must be made with metal,not a non conductive plastic. However, a silver coated plastic might work well as a metal in RF.