The only time BM and Kaido acted retarded was early fight (for Kaido), not crushing Luffy's skull after ragnarok, and the entire Law's exchange.
Zoro's Ashura and Hakai feat, as well as stuff like Tatsumaki, etc are irrelevant to any plot armor or Yonkos acting retarded.
Zoro warns Kaido even before his attack, then proceeds to blitz and permanently scar him.
Hakai comes, blocks it, creates a massive shockwave shocking both the Yonkos, and holds it momentarily.
Those two alone >>>>>>>>>>>>>> anything YCs have displayed. In fact, YCs have straight up shown to be significantly worse.
Unless your dumbass thinks Zoro casually tanking Indra's lightning bolt (literally the ONLY attack that even managed to hit him before Hakai) is plot armor rather than him just being strong