a face (that) only a mother could love
A face that is strikingly or shockingly unattractive. His physique was remarkable, but unfortunately he was cursed with a face that only a mother could love. She is extraordinarily bright, but she has a face only a mother could love.
face (that) only a mother could love
and face that could Stop a clockFig. a very ugly face. (Usually jocular. See also homely enough to stop a clock.) The poor baby has a face only a mother could love. Look at that guy. That's a face that could stop a clock.