Mrs. Fitts doesn't get a lot to do or say in the movie, and she's kind of a mystery. Most of the time, she seems pseudo-catatonic, just sitting around the house staring off into space.
Then, when someone like Ricky talks to her, she seems completely out of sync with her environment—she's spaced out and unaware of what's going on or how people are feeling. It's unclear what her deal is, but with a domineering husband like Frank Fitts, you kind of wonder if she didn't just withdraw all the way inward to avoid the fireworks that seem to result when you try to be yourself around Frank.
And hey, fair enough—as we know from what happens to Lester and Ricky, he's a dangerous guy.