"What a catch" in terms of dating is a saying that explains that a person has desirable qualities. The joke is referencing a dairyman's daughter. A dairyman is a person that sells dairy products and dairy products come from cows.  A calf is a baby cow. "What a calf" sounds close to "what a catch". The joke is saying that the dairyman's daughter is has desirable traits. 
She was only a Dairyman ' s daughter , but what a calf