If I’ve visited it before, and it’s usually okay, then I do nothing. And I will go back again. It’s almost certainly a one time thing, and something went wrong. If it’s consistently bad, then I don’t go back.
I might complain if they gave me one of those “tell us how we’re doing” tickets. And I would spread things by word of mouth. But I only complain if they got the order wrong or I suspect there’s something actually wrong with the food.