In what universe is spitting lava and acidic mist weak?Here is why. Look at her panels in all these pages.
You must be registered for see imagesYou must be registered for see imagesYou must be registered for see images
Her will was the weakest of all the Kages. She showed the most fear. And she was mostly just a support during the fight with Madara.
The ones that truly fought Madara all out with very strong courage were Oonoki, Tsunade and Ei. They were fearless. IMO, Mei had done the least during this fight.
Looking at her jutsus, I believe all the other Kages can beat her easily. Are her youton and acid mist enough to fight the other Kages?
Oonoki can solo her with his Jinton. He can fly to avoid Mei's attack.
Tsunade got her Byakugou jutsu, she can't be melted.
Ei got his incredible speed and strength, his taijutsu are powerful. He got Shunshin too.
Gaara got his ultimate defense, his sand.