I read the books so was very aware of that and the fact that she thinks she's as "clever" as tywin doesn't change matters. cersi thinking she was the manipulative villain of manipulative villaians, but in reality she was playing at saturday morning cartoon level in a game of grandmaster villaians more than happy to play the long game.
If cerci were a player no group would tolerate her for long without the gm pulling rank with "we are dating/engaged/married" & barring that particular shield of plot armor any gm equal or better than mildly incompetent would quickly boot her player or strike her dead with lightning/falling rocks long before the fall of kings landing. She actively backstabbed & pushed away people like jamie & tyrion who were keeping everything together If the other "PCs" are littlefinger & the queen of thorns though my point is proven given the completion of their plotting. A player who chooses to backstab the party & engage in constant campaign sabotage as Cersi lannister did. Your attempt to pin it on a gm needing to up their game ignores the fact that this one toxic player is only one player in a group of more than one. Part of being even a "sorta kinda almost ok" level gm is to make sure that a toxic player does not go around ruining the experience for the other players at his or her table.
edit: even with the tv series trying to suggest that she was sometimes mildly competent
her incompetence was blinding & I found that within seconds of looking