Yes on the passive sensors, no on the defenses.
In my carrier doctrine, if the carrier comes under fire, you have screwed up already.
The carrier´s offensive _and_ defensive means are its fighter group. If the fighters can´t handle a threat and that threat gets close enough to the carrier to shoot at it, the carrier is dead, no matter how much armor he mounts. And any ton used for armor is a ton _not_ used for more fighters.
The same doctrin asks for large passive sensors, however, so the carrier does not have to light up his actives and give his position away. Usually, I have dedicated fleet-scouts for that mission (again, to maximize hangar space). Once the general area, the enemy is in is known, the fighter-scouts are send in to have a look-see.
Looking at the Akagi class, those carriers have actually too much armor, for my taste. If halfing the armor would let me squeeze in another 3 or 4 fighters, I would do it in a heartbeat.
