How exactly you come to the point that Big Mom soul, as freak of nature, as you called her, is stronger, or better than Nami's soul? Like freak of nature is physical ability, not the soul thing. Maybe BM is experiment from WG, we still don't know the truth. Plus to that Big Mom actually have weaker soul - her tantrums, her main goal to became Caramel mother...like she tries to be another person. Is that you would call strong soul? Btw I believe that all souls are the same, but it could be that different people have different souls in type of their power. But Nami wouldn't lose here actually. Girl do things without power of natural born destroyer. Would be RinRin succesfull without her power?
Now talking about Nami's abilities, it was showed that Nami could encharge in seconds Zeus with power....same thing with prometheus, I'm like 100% sure. So with that she could put homies to their limits, while BM can't
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this answer for you aswell