He’s obviously not dim and I think I might surprise a few in saying I don’t even think he’s a prig or even that much of a classist. Whaa?
Aight. He’s not dim but he acts awkwardly but let’s have a look at his baggage. Simon’s been uprooted from not only his home and everything he knows but also from everything he had set out to do with his life. Keep in mind that at some point in very recent history, Simon not only came to the conclusion that his sister’s future had been borked by that he would have to bork his own plans (wife, family, career, etc.). That will mess you up and probably make you more then a little resentful of the 'verse.
Now let’s look at where Simon is the day he buys a ticket on Serenity. He’s essentially posing as a respectable individual with not-criminal cargo to move. No sir, nothing weird in that crate, why would the authorities be after me? Simon also doesn’t benefit from our narrative point of view knowing that Mal and crew are the good guys. To his point of view these are criminals that could as easily take his stuff and vent him out an airlock or sell him back to the Alliance. Sounds paranoid? Keep in mind, no one on Serenity is wearing their ‘crook with a golden heart’ badge.
Finally, Simon is the ultimate fish out of water. He hasn’t been trained as a social chameleon like Inara or to be unnoticeable like Book. To him ‘normal’ is the life he left behind but ‘normal’ is still what he’s accustomed to interacting with. Notice how when he looks down on things (the souvenir shop, the freak show alien) he always does so in a commiserating fashion that includes a member of Serenity’s crew. He’s not intentionally putting them down but trying to include them in his dialogue. He just fails at it.
Simon is to his sister what Mal is to his crew and while it doesn’t sit well with us how he treated Kaylee in order to evade the Alliance and did nothing to endear him, we wouldn’t think twice about Mal shooting someone to protect a member of his crew, even Jaine. Speaking of which, it’s worth noting how unfair it is to ask that Simon accept Serenity’s crew when there are obviously elements hostile to him in that crew until very late in the series. This is all disregarding what was said in the cast: someone like Simon isn’t just smart in a ‘knowing more things’ kind of way but also in a ‘brain wired differently’ way.
In my opinion…