Lara I sort of saw the excessive mentioning of brands as a very specific technique. To me, this was one of Bateman's coping mechanisms, his persistent focus on material things because they're concrete and real, as opposed to the detachment he feels when he's really losing touch with reality. Brand names were also used to reinforce everyone's focus on material things, and everyone's warped sense of what is important.
Hannah I guess that's really what the book is trying to satirize. The uppity women and their materialism. I thing the in depth descriptions of his clothes were supposed to emphasize his desire to fir in and how much detail he paid attention to when trying to create his mask of normality he hides behind.