I would honestly place her below Hybrid Kaido given what they two displayed
This premise collapses into itself if you put any thought behind it.
If this were true, there wouldn’t have been Four Emperors. There would’ve been three.
If Kaidou could defeat Big Mom that easily, he would’ve taken her for all she had years ago. He would’ve seized her lands, territories, treasures - everything. Including her Red Ponyglyph. Hell, if not him, Whitebeard or Shanks would’ve.
The reason none of Emperors were challenging each other despite not being particularly friendly or having outright animus for one another is very simple.
It’s the same reason why predators avoid fighting other predators or dangerous prey.
It simply isn’t worth the risk of death or injury.
It doesn’t matter if Kaidou is stronger than Big Mom if he can’t beat her without sustaining injuries that could create an opening for someone else to come for his head.
There’s a reason why the Four Emperors are considered a part of the power balance of the world. None of them is winning a fight against another Yonkou without sustaining serious damage and potentially even dying themselves, be it from the fight or the wounds that linger after.
It’s why they kept to their own lanes, rarely meeting in person. And it’s also probably why the marines and WG didn’t think they’d have to fear them teaming up until Big Mom and Kaidou did just that - which was of course cause for apprehension.