The mylar was for thermal purposes and not for radiation proofing. In space you have two choices. 1) lots of material with hydrogen to absorb particles without creating secondary radiation, or 2) worse choice, heavy heavy shielding with metals. The design choice is to use light materials that do not interact with radiation to create secondary emissions and instead allow for a small amount of particles to zap through you which is cumulatively less risky than being exposed to heavy, metal, walls that emit secondary radiation when a cosmic ray passes through. These days we in fact use polyethylene plastic in the Space Station for this shielding in crew quarters because it is loaded with hydrogen. You would not use poly on a nuclear reactor however, instead lots of concrete there.