It depends on what you consider happiness. If Happiness means constantly slightly elevated dopamine levels as produced by non impairing drugs, yeah it's not going to do a damn thing. If by happiness you mean fulfilled by the work you do, no marital trouble at home, no crippling financial issues or personal crises outside of work taking attention away or requiring effort that leaves the employee sleep deprived, then yes it will make a huge difference. Happiness doesn't have some special effect in and of itself, but it is an indicator that problems that can creep into work time are manageable or nonexistent and that the person is a least somewhat motivated to do their work, which will produce much better results than the opposite circumstance.