It's not cardboard--it's Kleenex.

Man of Steel--Woman of Kleenex.

The whole "world of cardboard" bit is an actual quote from Supes in JL Unlimited.

That man won't quit as long as he can still draw a breath. None of my teammates will. Me? I've got a different problem. I feel like I live in a world made of cardboard, always taking constant care not to break something, to break someone. Never allowing myself to lose control even for a moment, or someone could die.

they need a red sun lamp