Building off of @Rüdiger's answer, it means that Gloria thinks that Bill, Violet's father, is hot, as in very, very attractive.
On a scale of 1 to 10, 1 would be the worst and 10 would be the "best" or "perfect" quality (Wikipedia).
Think of this like the gymnastics competition in the Olympics where 10 is a perfect score.
In the meantime, Violet responds that her father is sometimes an eleven or a twelve to indicate that he's so good-looking that he has broken the scale (Definition Two).
And while Gloria had been judging based solely on looks (because as you say, he had been dressed up for the wedding), Violet is giving her rating to show her appreciation for her dad for his personality and as a person.