I get it that she traded off her sanity and mental capacity for gluttony-induced strength but fat Big Mom quickly became a one-joke-character when we got to Wano. These Rocks Pirate flashbacks show a different side to Big Mom that make her a more interesting and entertaining character to follow, she's obviously not a good guy pirate but she used to be a more level-headed woman than the obsese psycho granny she is today.
It'd be cool if the lava bath at Onigashima shaved off some pounds and knocked some sense into the woman. A mentally competent Big Mom would be the most terrifying emperor.
Yonkou had to feel larger than life, in particular Kaidou and Big Mom were abominations. That's why Oda introduced her as a complete disgusting monster.
This only works because it's after the fact. It's cool to see Big Mom when she was "normal" but there was no shot she would make the cut this way into the current timeline.
In a way these flashback characters really feel like they were playing house as Akainu once said about WB. There are pros and cons, the pros are the top tiers were actually interacting with one another unlike today, the cons are it feels mickey mouse, whereas there's a lot more weight to a top tier even making a minor move today, they're not just running around being buddy buddy with each other.