If Yamato's performance vs Kaido places her at low top tier in my tier list, I wouldn't later downgrade her because she fights weaker opponents.
Zoro faced a weaker opponent than Sanji in Punk Hazard.
I believe Fishman Island Zoro was well above Luffy (Zoro didn't grow significantly until Enma), yet Luffy got the stronger opponent.
Or, I just don't agree with this logic of fighting stronger opponents = stronger. And it's not something I've ever agreed with.
If Yamato is pushed by an opponent I believed to be weaker than her, then I might downgrade her. But fighting someone weaker than Sanji's opponent doesn't make her weaker than Sanji.
I didn't downgrade Zoro because he was given Monet. Zoro negged Monet, so Monet can't be used to scale him.
Even if Sanji had beaten Vergo in a very high difficulty fight but Zoro negged Monet it wouldn't be proof Sanji > Zoro.
Jimbe isn't going below Sanji just cause he's fighting Who's Who. It's because he's been fighting Who's Who for several hours and hasn't beaten him. I expect to end up scaling Jimbe to Who's Who or (at maximum) Jack level, while I expect to end up scaling Sanji to Queen level.
If Jimbe stomped Who's Who, I would have kept him above Sanji.
- Kaido has been repeatedly beating Yamato up since childhood.
- Kaido placed cuffs on her that will kill her if she left her prison.
- He was ready to kill her outright for defying him.
The idea that Kaido will not fight Yamato seriously is not currently borne out in the manga. Especially not when he's willing to kill her if she steps out of the role he's fashioned for her.
My take on Yamato joining the crew.