This excerpt from the article is relevant to your reaction:
In a letter to a student who pointed out that the chubby Midwesterners in his State Fair essay seemed “animal-like,” he answered, ashamedly, “It’s death if the biggest sense the reader gets from a critical essay is that the narrator’s a very critical person, or from a comic essay that the narrator’s cruel or snooty. Hence the importance of being just as critical about oneself as one is about the stuff/people one’s being critical of.”
What I'm getting at is that there were certainly times when DFW did say some pretty wise things, not because he was a wise sage, but because I think he sat and thought about certain aspects of life deeply. So, I'm not sure why you would have a one-sided impression of him as an asshole. Similarly, he certainly did write things that sounded like he was on the verge of wanting to go off on people, not because he was an asshole, but because he was clinically depressed. Anyway, I guess you've chosen only to remember one side of his personality, so it sounds a bit disingenuous. I don't expect people, especially in fiction, to have a flat-line, monotonous personality. A story, after all, needs to make things interesting and the mechanics of it require some tension.
We can sit and judge his personality when giving interviews, where most of the time he appeared pretty calm, maybe due to the fact that he was medicated--I'm not sure--but it would be a pretty useless exercise that kind of reduces everything to gossip.
His political opinions don't make him a worse or better writer. I mean, do semi-popular writers or artists have to always have a hard-lined opinion on something? He seems pretty liberal and, at the very least, not a right-wing conservative nutcase in the 2003 German interview on ZDF, if that helps any.