"A Confederacy of Dunces" struck me much more as a chronicle of weird people and their foibles with a large dose of regional flavoring--sort of like Napoleon Dynamite but focused on a city underbelly.

I don't remember "fatness" being a signal of evil, and I don't remember any of the non-fat people being any better than the fat ones.

However, it has been quite a while since I read the novel, and I really didn't like it very much. So, my memory may be faulty, and your mileage may vary.

The main character, Ignatius J. Reilly, was extremely fat, which was part of his ignoble caricature.

Baron Harkonen takes fat and evil to extremes.