Is Luffy One of the Four Emperors?

Is Luffy One of the Four Emperors?

Following the events of the Whole Cake Island Arc, the World Economy News Paper has referred to Monkey D. Luffy as standing adjacent to the Four Emperors as the "Fifth Emperor" of the sea.

Who's the weakest Yonko?

Shanks Shanks is the weakest Yonko.

Is Big Mom scared of Shanks?

TL;DR Big Mom is afraid of Shanks because he's just too powerful for her.