A dent is a section of head which has been made both longer and thinner than the surrounding areas by an impact (forgive the pedantry, I'm trying to establish a logic chain). The only real "fix" would be to compress the mylar back into shape, which is simply not going to happen, because it is not particularly malleable. Heata makes sheets of mylar thicken and contract, like a shrinky-dink. The lighter would likely thicken/shrink the surrounding area with the dent, stretching the dented mylar flat(ish), which would create a couple different thicknesses, and probably a brittle spot.
Totally not worth it, IMO.