No, not a joke. Whatever you think of Ellis’ writing (and I think it is indeed tedious and puerile myself) he is going for a certain effect. When, as mentioned by another poster, Patrick Batemen goes on a seemingly endless spiel of commercial products, Ellis is making a commentary on the vacuousness of the 'Eighties consumer culture, and Batemen’s psychopathic behavior is supposed to be an extension of this. Now, whether this appeals to you as cogent literary content, engaging wordsmithing, or pathetically self-involved stylings is a matter of personal preference, literary exposure, and experience.
While I won’t defend the style of Ellis, similar criticisms are often advanced of the writing of Joseph Heller, Thomas Pynchon, and David Foster Wallace, who are all favored authors in my collection. I will admit, though, that my first reading of all of these authors was less than celebratory; in the case of Heller, it took some amount of life experience before I could appreciate the dark humor of Catch-22 and Something Happened, although his other subsequent works came easily (except for Closing Time, which was clearly a second-rate work of an author at the end of his career). This “type” of writing is intended to challenge the reader, and cannot be compared in terms of entertainment with enjoyable but predictable and formulaic genre writing. It isn’t about plot and sympathetic characters; it is about tone, style and effect. The best writing (in my opinion) manages to do both–see Don Quixote or The Honorable Schoolboy–but not all authors aspire to or intend that.
Whether the o.p. wishes to stretch the bounds of her literary exposure that the peril of wasting a few hours on an ultimately unsatisfying book (which I think American Psycho is) or reject it in favor of something more palatable is her own choice, but it is worthwhile to pick up a book once in a while that is difficult, challenging, or disagreeable just to see if there is something more to it than a superficial, plot-centered read would suggest.
Stranger