This is a really interesting question to think about. I guess if you want to take this line you can definitely argue that Mary Maloney is "caged," however her cage is of a very different nature than the cage of Minnie Wright. The beginning of "Lamb to the Slaughter" makes it absolutely clear that Mary Maloney is completely obsessed with her husband. Consider the following quote:
For her, this was always a blissful time of day. She knew he didn't want to speak much until the first drink was finished, and she, on her side, was content to sit quietly, enjoying his company after the long hours alone in the house. She loved to luxuriate in the presence of this man, and to feel--almost as a sunbather feels the sun--that warm male glow that came out of him to her when they were alone together.
This is just one example of how Mary Maloney has assumed a completely servile role to her husband. Note how she almost basks in front of him, comparing herself to a sunbather. It appears that her entire reason for being is dependent on her service and love for her husband. She appears to have no identity outside of that role. Feminists would argue that this would be just as much of a "cage" as that of Minnie Wright. The only difference is that Mary Maloney does not know it is a cage.