I mean, yes, we know zombies are mindless. The brain's higher functions are utterly destroyed - along with personality, and anything you might call emotion. So they can't be "happy."
But what if they just seemed happier? What if, as they shambled forward in stiff mobs bent on grasping, seizing, biting into warm, breathing flesh; tearing and gulping and gnawing with insatiable maws until whatever's left of their victim blinks its last blink and rises again to join in, what if as they went about, slowly decomposing yet ever-intent upon their grisly business - what if they were smiling? What if their eyes held some unchanging semblance of pleased surprise at all times? What if their blood-chilling groans were instead, warm sighs of relaxed contentment?
I bet that would go a long way toward solving some of these image problems.