Smithy Ide is hands down the most self deprecating character I have ever read, and his continual castigation of himself has quite a few effects on the story. I don't find it to be particularly endearing. However, it does factor into the reader's opinion of Smithy in the sense that his self deprecation is a part of the whole Smithy-to-us vs. Smithy-to-the-rest-of-the-world dichotomy, which is a thread that runs through the whole book.
Every time Smithy says or does something stupid, he mercilessly ridicules himself for it, and, as readers of his thoughts, are privy to his private little hatefest. Whichever other characters witnessed his screw-up don't see him upbraid himself. All they see is an obese man who with seemingly sub-par social skills. We, then, see that he does in fact understand social rules and recognizes when he is awkward, other characters don't.
The self-deprecation also complicates the issue of narrator reliability. On one hand, Smithy's self-deprecation could make us trust him. Maybe it's because he seems to trust us, or maybe it's because of the fact that he is willing to portray himself in such a negative way. I've tended to trust him.
But sometimes I wonder if he doesn't become blinded by his own self loathing. For example, when he talks about how his shorts were getting looser, it never even occurs to him that he might be losing weight, which is odd, considering how fixated he is on his size. Of course McLarty fully intended for the readers to pick up on that bit of subtext, but maybe Smithy's behavior isn't always as idiotic as he makes it out to be.
This little concern is kind of hard to address because Smithy's narrative style. Often, his descriptions don't leave a lot of room for subjectivity: "I was 279 pounds." He gives a pretty succinct accounts of his dialogue and actions. But sometimes this succinctness (not a word?) works backward, allowing ambiguity. He assumes he seemed stupid when he said/did such and such. Do we believe him?
Smithy's lack of self-confidence is a really interesting and unique character trait, so I'm excited to see what McLarty does with it as Smithy makes his way across the country and confronts the issues from his past.
Fun fact: I googled Ron McLarty, and it turns out he's an occasional TV actor and has appeared in Law and Order and did a voice in a Batman video game. Huh.