"But but Sanji is weak because he's afraid to fight Big Mom with Luffy."
Meanwhile, Zoro out here saying that if Big Mom joins then it will be a huge pain in the ass.
Whatchu mean?
Sanji make a cake for Big Mom because she's hungry. Regardless of enemy or ally, Sanji will help those in need because he doesn't want Big Mom to end up like his childhood of starvation. And Big Mom was never going after Sanji in the first place, Luffy declared war against the Big Mom Pirates in Fishman Island.
Yes, that's how Sanji became the World's Strongest Swordsman.