i dont think big mom is deafeated, if we compare her with same df user like her, moria and sugar. When moria and sugar ko or lose, the df effect to the victim is back to normal. So if big mom ko, nami will lose zeus, and i dont think oda will let that happen. And i think big mom is the weakest younkou. When luffy hit big mom using G4, she has to defend herself using armament haki. And Kaido didnt use any haki to defend himself when luffy hit him with G4 the first time. And big mom has lower IQ too. She solely depend her ability to survive with her brutal strength, and it shows time and time again, like this chapter. For me, big mom as a younkou is compareable to moria as a sichibukai. Not in stregth, but in overal fighting style and IQ. Brute strength but low IQ.