So is the belief that Mercer gets to spend 40 hours a week preparing for his game? Because that is not true. The man has two careers outside of planning for the game. One with his leadership role with Critical Role (which includes participating in a lot of other gaming events, etc...) , and the other with his voice acting career. He is likely as busy, if not busier, than most of us.
He does have additional resources that other DMs do not have, in the work of wikis for his world and a plethora of online summaries of his world. He has mentioned that he occasionally Googles to find out what he previously said in a game... And he has recordings of all of his streamed games to refer back to when he wants.
However, all of that is fairly irrelevant to what makes Mercer a great DM.
He loves his world, and knows it back and forth. When he adds something to the world, he cares about it and builds it up so that he can use it easily.
He understands story structure. He doesn't just drop a bad guy into the game - he builds the story around his bad guys. They have a reason to be there. They have goals. They have personality. They are actual characters.
He understands the role of the protagonists in a story. The story is about the achievements of his PCs, so he creates a world in which they can achieve. He puts hurdles in front of them that play to their strengths. He does not lament a high AC, a high damage capability … he celebrates it with the players. Vax was super fast - and he made a lot of opportunities for Vax to use that speed to great effect. He roots for them and creates opportunities for them to best the challenges that only they could face.
He is great on the fly. If you think preparation makes Mercer great, you are not watching him respond when the PCs do something unexpected. There are times when they pull the rug out from under him in ways he never expected. If you watch his face, you can see those moments of loss when, as a DM, a carefully laid plan and storyline is obliterated by a cupcake. You can see him create NPCs off the cuff when the PCs go someplace he only roughly fleshed out previously. You can see him make quick notes as he goes about the ideas that occur to him as he introduces a new NPC that - 4 seconds earlier - did not exist.
Does he do all of this perfectly - every time? No. There are times when human nature gets him and he gets frustrated that Caleb remembers everything, or that Beau stuns his bad guy before they do something. As a DM, when we take pride in a plan or a designed encounter and the PCs cut it short, it can be frustrating. However, he is really good most of the time and - in my book - is the best DM I've seen in 40 years of D&D.
If you think Critical Role is a waste and not worth watching, then you're missing out. You don't need to watch the entire series, and you don't need to emulate it, but if you think there is nothing you can learn from Mercer that would make your games better, you're not paying enough attention.