When we say directly responsible, we mean he provided the financial backing and support to put films like Suicide Squad, The Intern, Pan, and the Entourage Movie out into the world.
Sure, he wasn't in the editing suite making sure that Suicide Squad was a pile of dogshit and he wasn't on the set of Get Hard writing all those horrible rape jokes either. However, Steve Mnuchin's side business, Rat-Pac Entertainment, was directly responsible for funding these films and now he's in charge of the US economy.
As well as being a prolific movie producer, Mnuchin also foreclosed on over 36,000 homeowners and was described as a "foreclosure machine" by housing advocates. As for his policies, it makes about as much sense as the third act of Batman v Superman: Dawn of Justice (which he also produced).
Mnuchin, through Trump's policies, wants to repeal the Dodd-Frank law that places regulation of the financial industry in the hands of the government and enact an insane border tax on good produced outside of the US.
In his speech announcing Mnuchin as Treasury Secretary, Trump said that the executive producer of the Vacation remake - which currently sits at 26% on Rotten Tomatoes - would "fight for middle-class tax reductions, financial reforms that open up lending and create millions of new jobs, and fiercely defend the American tax dollar and your financial security."
Think about it - do you want the man who was responsible for Jared Leto playing The Joker in a high political office?