It sounds as though "just mean" is an obvious impeachment in your view, and if your position is that we shouldn't write and publish mean things, then...well, that's fair.
I'll agree it's mean. But in many ways I think the piece teaches worthwhile lessons by being very specific, which may also be excessively personal. For example, the novel idea of the author occupying a reality that "is not just un- but anti-real" doesn't land without the preceding litany of cliches. That sets up the next couple pages, which asks how we should read this sort of unreliable narrator, and proposes the explanation that maybe she's just that stupid. You objected to this—the "frontal lobe activity" bit—but so did DFW, rejecting that explanation and calling it "literally incredible" then proving it with examples.
I don't think that arc makes much sense in the abstract. And the specificity is even more necessary in the next move, from page 148 to 151, making the larger point about the depth and potential that existed in her specific story. And that, of course, gives us a grounding to consider the broader questions: DFW acknowledges that "neither Austin nor her book is unique" and turns to the central question: why the fact that athletes are "stunningly inarticulate" is "always so bitterly disappointing," which is about the genre.
The conclusion—the last four paragraphs of the piece—isn't just dusting over the meanness. The possibilities discussed on the last page are real ones and also deeply troubling to intelligent people, DFW included, who derive a lot of self-worth from their "interior struggle."