Oh, Mrs. Miller. You can think of her with wincey pity every time you see someone's mom running after the school bus in a nightie waving a bag lunch or dumping the whole caddy of sugar packets from the restaurant into her purse.
In a time when the role of a wealthy young lady's mother was to steer her through the murky waters of social decorum into the serene port of marriage, Mrs. Miller is like a rowboat with one oar and no compass.
It should be noted that she's always well dressed, despite her social snafus, but we get the sense that this is Daisy influencing her mother's style rather than the other way around. In fact, in one scene, Mrs. Miller is wearing her daughter's shawl—a clear symbol of the role reversal that contributes to Daisy's downfall.
Mrs. Miller tries, she really does, but she's always doubling back on her own ideas of what's right and what's wrong, usually out of sheer disorganization and absent-mindedness. For example, when relating Daisy's deathbed message that she had never been engaged to Giovanelli, Mrs. Miller adds, "And then she told me to ask if you remembered the time you went to that castle in Switzerland. But I said I wouldn't give any such messages as that" (2.260). D'oh. You totally did give that message anyway, Mrs. M. Come on, keep it together.
The narrator likes to point out Mrs. Miller's "much frizzled hair" (1.164) to emphasize her ridiculousness, in the same way that the awkwardness of Liz Lemon's youth is always enhanced for us by that crazy wig Tina Fey wears in the flashback sequences.
Winterbourne, in his last encounter with Mrs. Miller, "paid her the compliment of saying to himself that she was not, after all, such a monstrous goose" (2.260). So, basically she initially comes off as a monstrous goose, and then decreases slightly in goosiness over the course of the novella to end only somewhat mildly goosey, but not monstrously so.
That Winterbourne is such a flatterer.