The saying of "she could stop a clock" means that she is very very beautiful that it can make the clock stop.
She could stop a clock can mean that she is very beautiful although it can also be used to mean that the girl or woman is ugly that the clock would break and stop when she looks at it.
Have a face that would stop a clock means to have a very ugly face and be extremely unattractive.
Having a face that would stop a clock always used to mean ugly and not beautiful and it sounds dated although it is still very understandable.
When someone says that someone else has a face that would stop a clock it implies that the person's appearance is so unpleasant or startling that it could theoretically halt the movement of a clock's mechanism.
The phrase a face that would stop a clock is often used humorously or unkindly to someone to describe a persons appearance being ugly and unattractive.
The phrase of have a face that would stop a clock is thought to be from the early 20th century, with the idea being that the face is so shocking that it would cause the mechanism of a clock to fail.
It's mainly associated with unattractiveness, although some people interpret it as being a way of saying that someone once had a strikingly beautiful face.