Buddy's first impression of the Matron of Honor gives us a pretty good idea as to what her character is going to be like. "She was a hefty girl of about twenty-four or -five, in a pink satin dress, with a circlet of artificial forget-me-nots in her hair," he narrates. "There was a distinctly athletic ethos about her, as if, a year or two earlier, she might have majored in physical education in college. In her lap she was holding a bouquet of gardenias rather as though it were a deflated volley-ball" ("Roof Beam" 2.17). Added to this physical description we get a sense of her personality: she is blunt, bulldozer-y, and lacks grace (social, verbal, or otherwise).
On the other hand, we get that the Matron of Honor is ticked off. Her best friend (Muriel) just got stood up on her wedding, and now the groom's brother is sitting in the car, on the way to the reception, pretending nothing is wrong and making no attempts to apologize, justify, or explain his brother's inconsiderate behavior.
Not that Buddy really could if he wanted to. The constant clash between Buddy and the Matron of Honor emphasizes one of the key difficulties for any member of the Glass family: communicating with those outside the inner circle. We talk about this more in Buddy's "Character Analysis," but the short version is that the Glass family is so unique and so insular that they don't understand, and can't relate to, anyone outside of it. Similarly, outsiders can't understand the Glass siblings either.
The character of the Matron of Honor also brings us to a sticky point for many readers and critics. In his writing, Salinger clearly reveres the Glass family and derides those who are not part of it. This particular character, the Matron of Honor, is caricaturized to the point of being ridiculous. Her gracelessness and superficiality are exaggerated while the characters of Buddy and Seymour are held up as an ideal of mental and spiritual grace. Many critics are bothered by this authorial attitude, feeling that Salinger goes too far in worshipping his creations and knocking "the general public."