IMHO, it's a mediocre place if you get mediocre service. It's up to the boss to motivate workers to give 100% service 100% of the time to 100% of the diners, regardless of "good night/bad night" personal life considerations. A well-run place doesn't have mediocre service, won't tolerate it, and if they're on the ball, they'll catch it and nip it in the bud by coming around and asking if everything's OK.
However, what if it IS really <<a nice neighborhood restaurant with an excellent menu a fine chef/cook and just some poor schlep waiter having a bad night?>>
In that case, I'd call over the owner (or the manager if the owner is not around) and let him or her know that the service was a bit off that night. Tell him exactly how, just so there are no misunderstandings and he doesn't over-react on the waiter. Odds are, they'll make it up to you. As long as they acknowledge the beef and are prepared to take some steps to make it up to you, that's good enough for me. The owner gets a chance to correct falling standards, which benefits us all, even the mediocre waiter, I get my concerns addressed in a tangible and satisfying way, the waiter gets his mediocre tip for his mediocre service, and we're all friends.