Do you mean right at the beginning of the story -- before we know what happens?
I suppose that if that is the question I would say that she seems like a stereotypical old time housewife. She is pregnant, she seems devoted to her husband. All she is doing is waiting for him to come home so she can see if he needs her to do something for him. She has all the stuff ready so he can have a drink right away. She is happy with every minute that passes because it means he will be home sooner.
So I would say that she seems like something out of a 1950s TV show like Leave It to Beaver.
I wonder whether she was already aware that something was wrong. Is she a bit too perfect? When re reading it she sometimes reminds me of Antony Perkins at the end of 'Psycho' when he maintains the helpless old woman persona who wouldn't hurt a fly. When Mary picks up the lamb joint she grips it in two hands like a baseball bat. Am I too suspicious?
She definitely does not give me the impression that she is capable of murder. Mary is tranquil,placid and in love with her husband whom she would never hurt. Until Patrick mentally murders her. As you can see she went a little mentally unsound after he breaks the news to her. (:
she starts of as a sweet lady, who loves her husband, and loves her life.
But as the story progresses she becomes a cold calculated killer.