The male stereotype of women in "Lamb to the Slaughter" portrays females as weak, co-dependent, and submissive individuals that would have embodied the proverbial "barefoot and pregnant" paradigm.
For starters, it is a clever technique to apply those stereotypes to Mary, in the first place. She was indeed pregnant, she was dependent on her police officer husband as her provider, and she was also submissive to his wants and needs.
That aside, these very traits rendered Mary as someone who was feeble, susceptible and vulnerable in the eyes of society: a young, pregnant, police officer's wife.
All of these traits are tantamount to innocence, by default, in the eyes of men who are good, but also somewhat primitive, according to their indirect characterization.
Mary may have not suspected (or she may have, we will never know as readers) what her husband was up to. However, she was quite aware of the behaviors and dynamics of her husband and his peers. This is what led her to use the stereotypes that men had on women all to her advantage. This was her salvation after the murder. Give society the optics it expects to see and the rest will take care of itself:
she told herself as she hurried back, all she was doing now, she was returning home to her husband and he was waiting for his supper; and she must cook it good, and make it as tasty as possible because the poor man was tired;[...]Mrs. Patrick Maloney going home with the vegetables on Thursday evening to cook supper for her husband.
That being established, she acted the part perfectly. The men were too blindsided by the mundane aspects of the issue at hand (the morbidity, their own hunger, their pain) to really see the cues that rested right in front of them. They immediately assumed that Mary was innocent simply because she fit the parameters of the "innocent" woman established by the stereotypes of the time, which were described above. This is how she used them to her advantage. She was, in the end, the brightest of the bunch.