Why wouldn't you blame him? He's the one that spent most time working on that movie than anyone else on the planet. Not to mention, unlike the Ayer case or the Snyder case or the DeCosta case, there are absolutely zero reports, leaks, or crew testimonies of any kind to suggest that control got taken away from him at any point during production, or that he was forced to do very extensive reshoots, or that he was rushed, or anything of the sort. He also proudly boasted about how the cameos were his idea. The only scene that was ever reshot was the Clooney scene, which was brilliant.
Mamá was a dog**** movie too so I don't know why it's unbelievable to accept The Flash was bad because he's a bad director. He is a bad director.