Bessie doesn't really seem as though she's the Glass children's mother. She doesn't share their cynicism, negativity, tendency to judge nearly everyone, verbal sparring skills, spiritual bent, or aesthetic eye. She is, instead, a compassionate, Irish-Catholic ex-vaudeville performer.
But for everything she lacks, and for all her differences from them, Bessie Glass certainly understands her children in a way that no outsider ever could. And her children both recognize and value this. Check out this passage for a concrete example:
Mrs. Glass directed a long and oddly comprehensive look at his profile. […] "You either take to somebody or you don't. If you do, then you do all the talking and nobody can even get a word in edgewise. If you don't like somebody – which is most of the time – then you just sit around like death itself and let the person talk themself into a hole. I've seen you do it."
Zooey turned full around to look at his mother. He turned around and looked at her, in this instance, in precisely the same way that, at one time or another, in one year or another, all his brothers and sisters […] had turned around and looked at her. Not just with objective wonder at the rising of a truth, fragmentary or not, up through what often seemed to be an impenetrable mass of prejudices, clichés, and bromides. But with admiration, affection, and, not least, gratitude. And, oddly or no, Mrs. Glass invariably took this "tribute," when it came, in beautiful stride. She would look back with grace and modesty at the son or daughter who had given her the look. (Zooey.5.33-4)
In fact, the more deeply we look into Bessie's character, the more we realize that she's not just a simple-minded person that Zooey's teasing comments make her out to be. Take her kimono, for one. Sure, Buddy takes a shot at her clothing by calling it her "pre-notification of-death uniform," but it actually goes a long way in establishing Bessie's character (Zooey.4.20). The narration tells us that it:
serve[s] as the repository for the paraphernalia of a very heavy cigarette smoker and an amateur handyman; two oversized pockets had been added at the hips, and they usually contained two or three packs of cigarettes, several match folders, a screwdriver, a claw-end hammer, a Boy Scout knife that had once belonged to one of her sons, and an enamel faucet handle or two, plus an assortment of screws, nails, hinges, and ball-bearing casters." (Zooey.4.20)
Mrs. Glass comes prepared and ready to fix, adjust, and tweak just about everything.
There's a real depth to this woman. The other housewives in Franny and Zooey spend their days shopping at Lord and Taylor's. Mrs. Glass, on the other hand, "looked, first, as if she never, never left the building at all, but that if she did, she would be wearing a dark shawl and she would be going in the general direction of O'Connell Street, there to claim the body of one of her half-Irish, half-Jewish sons, who, through some clerical error, had just been shot dead by the Black and Tans" (Zooey.4.20).
Which reminds us of the death of two of Bessie's sons, an element of her character that the text never for a moment lets us forget. Passage after descriptive passage drives home the tragedy of Seymour's suicide and the weighty impact it's had on this household. Consider this as a prime example:
It was a very touch-and-go business, in 1955, to get a wholly plausible reading from Mrs. Glass's face, and especially from her enormous blue eyes. […] Once, a few years earlier, her eyes alone could break the news (either to people or to bathmats) that two of her sons were dead, one by suicide (her favorite, her most intricately calibrated, her kindest son), and one killed in World War II (her only truly lighthearted son). (Zooey.4.73)
We can start to understand, then, Bessie's overbearing concern for her surviving children – especially Zooey and Franny, who are, after all, the babies of the family.