Whether adding unreliable components increases the overall reliability of a system depends on whether the system requires all components to work (in which case adding components can only make matters worse) or only some (in which case adding components can improve redundancy and make it more likely that the final result is correct).

In the particular case of spotting mistakes made by ChatGPT, a mistake is spotted if it is spotted by the human reviewer or by the critic, so even a critic that makes many mistakes itself can still increase the number of spotted errors. (But it might decrease the spotting rate per unit time, so there are still trade-offs to be made.)