do you ever judge a restaurant by the cleanliness of the bathrooms??
would you go back to a rest if the facilities were out of an Indian slum??
One has nothing to do with the other (assuming you mean that if the bathroom is dirty the kitchen must be, too)
If the bathroom was filthy, I would worry about the rest of the place. If the bathroom is also used by the staff, I would consider it critical. I was in a fast food place and after finishing my meal went into the bathroom--used by everyone--to discover there was neither hot water nor soap at the sink................so where are hands being washed?
I take the view that the toilets are public areas, on show to the customers. If they are dirty, then the standard of hygiene in the areas unseen by customers (like the kitchen) are likely to be even worse.
The only exceptions would be street kitchens throughout Southeast Asia and India because otherwise I'd have starved to death by the end of the trip.
But in any reasonably developed areas there's no excuse for a restaurant not having a clean bathroom.