I don´t know much how strong Big mom was back then, she was pregnant to that time.
The question is if Big mom was stronger back in that time to her current form, probably yes or not, we gonna see it in the flashback.
If her EoS was back then, I think she was easily a mid-high top tier, yet weaker then Prime Roger/Prime Whitebeard, don´t know how much gap was between these guys. I mean I always though Oden was legit a monster and mid top tier, but he get easily washed by Roger who didn´t even fight seriously. In that case I want to see how Oden battle on Kaido, then I could make a clear point between these monsters.