I think this relates to Diane’s “I don’t really believe people have a ‘deep down’” speech. He is exactly what you described, a goofball who’s good at making corny puns and is occasionally very incisive about how people relate to each other. He’s not self centered exactly, but he does whatever is going to make him happy, and avoids over thinking things. He likes positive attention so he comes off as a lot nicer and considerate than Bojack does. That’s his whole friction with Dianne, that the things he does for her aren’t really for her, they are to keep her attention on him, and that makes him incapable of really nurturing other people. I think that’s why he’s so desperate to be friends with BoJack, why he’s the one who picks him up from jail to go to the wedding. He see’s how similar they are in that regard.

He’s not smart, he’s clever. He’s just a mechanism that happens to extremely well suited for his environment. He doesn’t have to be introspective, there isn’t a secret plan or calculations about appearing dim to put people at ease. He just does what feels right and it works out for him, kinda. Like his whole relationship with Todd, they spend a ton of screen time together but they aren’t really friends in any meaningful way, hell their is an explicit joke that their relationship is just "Yes, and"ing each other.

Man even the V neck vs crew neck discussion fits with this. It’s not a big deal to him because he’s worried about his visual portrayal, it’s important to him because he likes things that are comfortable, he finds V necks comfortable, so his character should be wearing a V neck in the movie. Simple as.