The notion that Krugman is stupid assumes that he believes the drivel he spouts. You don't assume that defense attorneys believe what they say in the courtroom when they defend their clients, do you? Same deal with Krugman.
Krugman owes his position and income to being an effective spokesman for socialism, a system that benefits the "connected" elites that keep him in his position.
I believe he is stupid because I think he himself BELIEVES the tripe he opines on.