> Well if the outer layer does not deform that's not generally bad, no?

Yes it is.

Crumple zones are designed to both avoid going straight through through whatever they hit (which is usually an other car, though avoiding pasting pedestrian is also a good idea if you can), and disperse the energy of the collision to provide progressive deceleration.

Rigid frames are the original state of cars design, and it was bad, because when the car does not absorb the impact and progressively decelerate (again thanks to crumple zones) all the force of the crash ends up with the vehicle's occupants. Which is the exact opposite of what you're looking for: you want to sacrifice what's outside the safety cell in order to limit or nullify damage inside the safety cell (companies are even working on "sledding" the survival cell and adding dedicated shock absorbers, especially in smaller city cars where there isn't much space for crumple zones to do their work)./