And even in some cases, he's just insufferable and hard to please . . . granted, Grover does often get almost everything wrong, and his irritation is justifiable, but there are times when Grover can actually pick up on said irritation and actually return the snark (mainly whenever Frank was performing Grover) - like in the picture frame one where Mr. Johnson is losing his cool over Grover's continual use of the wrong shaped frame, at one point, Grover defends his gaffe with, "I was just BEING CREATIVE!!" If Grover had teeth, he'd darned-near be speaking through them clenched in the Speedy Pizza insert when he strains, "What-do-you-want-sir?!" through an almost closed mouth.
Again, in cases like this, Mr. Johnson's irritation is justifiable because Grover doesn't listen to him, which he more than strains himself in explaining to Grover almost every time something like this happens.