Why too little armor was still better than none

Can light armor make a cruiser worse by detonating shells that would otherwise pass through?

Not at the protection levels that mattered for cruiser combat, because six-inch, eight-inch, and even destroyer shells could still cause crippling damage to a very lightly built hull.

A cruiser would have to be built extraordinarily lightly for a six-inch shell to cross the entire hull without serious effect. A battleship-caliber shell might overpenetrate a small target, but cruisers also faced six-inch, eight-inch, four-inch, and 4.7-inch fire. The early Condottieri ships demonstrate the danger: concentrated destroyer gunfire could tear them apart before torpedoes finished the engagement. Limited protection therefore addressed common threats even when it could not stop the largest shells.