Unusually for a woman at that time and place, the narrator’s mother has achieved some measure of financial independence. However, this has only been achieved by murdering people for life insurance money. This includes former husbands, orphans that Mama has adopted from a New York social organization, and immigrant men, mainly Swedes and Norwegians that she’s managed to con into joining her in a farm partnership in the Midwest.
Over time, these men, like her ex-husbands and those poor, unfortunate orphans Mama adopted, vanish one by one in mysterious circumstances. And with each vanishing, Mama’s bank account continues to grow. Mama goes about her murderous work with complete insouciance, seemingly unaware that she's doing anything wrong. As far as she's concerned, she's simply looking out for her own best interests. After all, no one else is going to take care of her, so she might as well take care of herself as best she can, even if it involves stealing, conning, and killing.
The implication of Mama’s murderous rampage is that an obsession with achieving the American dream is dangerous. While not everyone who pursues "the dream" will end up as a serial killer like Mama, at the very least they’ll have a warped value system that puts the acquisition of wealth ahead of decent moral values.