That's a great question. I would say that there is a theme in the story about how things are not always exactly what they first appear to be. I don't think that means people or items always intentionally deceive people. I think Mary and the leg of lamb clearly illustrate that there are layers hidden beneath people and items that are not always first considered.
Mary begins the story as the quintessential doting wife. She lives for time with Patrick. He is practically the air that she breathes. But then Patrick tells her about the divorce. Mary operates on automatic for a bit, and then she kills him. I don't think it was intentional, but her transformation afterward is what illustrates the theme. She goes from being a devoted, innocent wife to a woman that plans out each step of her alibi and the destruction of the murder weapon. She even giggles at the cops' inability to figure out what she did.
"It's probably right under our noses. What do you think, Jack?" And in the other room, Mary Maloney began to laugh.
That theme of hidden layers within characters and items is also illustrated by the leg of lamb. At first it appears to be nothing more than what it is. It's a leg of lamb. Mary plans on using it for dinner. It's food. It's nourishment. It's not a weapon. But all of a sudden the leg of lamb becomes an instrument of death. Essentially, the leg of lamb deceived the reader the first time it was mentioned in the story. Later, it deceived the police officers too. They saw it as nothing more than a meal.