In the Da Vinci Code, Brown is not interested in art for art's sake: in other words, he is not primarily interested, for example, in The Last Supper as an artistic masterpiece. The aesthetics of a work of art are largely irrelevant to the novel. In the novel, art is primarily important because it contains coded messages that show that Jesus married Mary Magdalene. Certain pieces of artwork show that that the term "holy grail" is coded language that conveys a message about the Magdalene marriage. Certain works of art or architecture contain a secret language understood only by those in the know, such as the freemasons.
The Last Supper is a prime example of a painting encoding a secret message. The novel analyses the painting only to show that a disciple seated near Jesus has such feminine characteristics that she must (in the novel's universe) be Mary Magdalene. Likewise, that this figure is leaning away from Jesus to form a "V" symbolizes the holy grail, or the marriage, to those who know how to read its signs. If the painting were aesthetically a piece of trash, it wouldn't matter: the message it conveys is what is important.