There is a reason Oda has set Kaido to be the first top tier to fall. Kaido lacks versatility which is why he will he eash to beat. Oda never made luffy fight Big Mom because she is unbeatable. Her versatility is too strong for any top tier. Big Mom is unbeatable. Also other top tiers have more versatility than Kaido. That is why he is weak compared to Big Mom. Roger even ran away from Big Mom.