The hysteria is self perpetuating when it interacts with her actual gameplay.
Group x (the smallest) claims she’s useless, group y hears about. Group x and y play her less. Group z sees that she’s played less and joins group x in proclaiming uselessness. Group a points out some arbitrary statistic, which validates x y and z. and around and around and around it goes. This isn’t exclusive to mercy either.
Overbuff plays a large roll in this. Think social media when it comes to socio political issues. It’s presence just makes things worse.
On top of all is the brazen lack of objectivity, and overt ignorance. But I dont want to detail that and create any further animosity, it’s there and it’s glaring.