Why light carriers mixed naval and merchant standards

Should modern carriers be built to standards resembling merchant ships, as wartime light carriers were?

Only selectively. Wartime light carriers mixed naval and merchant practice because numbers provided replacement capacity, whereas a small modern carrier fleet cannot absorb losses in the same way.

The light fleet carrier as a deliberate mixture rather than a merchant ship merely fitted with aircraft. Naval standards were retained where combat requirements demanded them, while merchant practice simplified less critical parts of the design. Wartime planners could accept that compromise because more hulls were expected to follow if one was damaged. A modern navy with few carriers has less redundancy and therefore stronger reasons to build each hull to more demanding survivability standards.