Big Mom would take over Wano in less than 1 day probably
I think even Big mom couldn´t take over Wano without any help as Kaido did of Orochi.
Probably she can but the lost of manpower would be so much, it would make her far weakened. Overall it is like Wano is a yonkou lvl crew on his own, with Oden beeing a top tier as leader.
In the past it shows that Wano had at least everytime one top tier who cares about the land:
Ryuma
Oden
Future Momonusuke and Zoro(if he stay in Wano after One piece is finished).
I just wait to see more of Oden power, but judging by the hype around him, even the Navy would need a Admiral to take on Oden and it would be very close battle. Ryuma was even most like a step above(High Top Tier or even Top Top Tier).
All respect to Big mom, but taking on Wano is really hard and Kaido did it after making a plan who work over years to final get the result.
Wano isn´t like Dressrosa/Alabasta where pirates easily overcome their millitary force.
Because that I love Wano as Country or Elbaf, where they have such a great millitary force that Pirates don´t have even the face to encounter the lands. I doubt someone as Doffy would ever try to control Wano or Elbaf, he and his crew would get stomped very hard. Same goes for Crocodile and co. It make countries as Wano or Elbaf very great overall.