Carrier armor layouts are calculated risks

Would Taiho have been better protected by a uniform 3-inch armored deck and 6-inch belt?

Not necessarily; the answer depends on displacement, stability, cost, completion time, and which risks the designers were willing to accept.

The proposed uniform armor scheme is not automatically superior. Any redistribution changes displacement and operating characteristics and may delay or increase the cost of the ship. Truly impenetrable protection would require far greater thickness, so every practical carrier layout represents an accepted balance of vulnerability and capability.