OP here (though I don't claim any special insight, as I said).

It would be interesting to consider the differences between the Charlie Brown and Arthur Dent character archetypes.

One difference seems to me exactly the undying earnestness and optimism you mentioned: in a way, Charlie Brown and other American characters like him are simply not touched by failure (even if bad things happen to them), because of their optimism[1]. This makes them lovable: we appreciate them for this quality that we (most of the audience) do not have.

[1]: (or lack of self-awareness, in some other cases mentioned here like Homer Simpson or Peter Griffin)

Arthur Dent, on the other hand, is not gifted with undying optimism. He's constantly moaning about things, starting with his house and his planet being destroyed. This makes him relatable more than lovable: he's not a “lovable loser” (and for the right audience, does not seem a “loser” at all), he is just us, “my kind of guy” — we feel kinship rather than appreciation. We relate to the moaning (if Arthur Dent were to remain unfailingly optimistic, he'd be… different), whereas if Charlie Brown were to lose his optimism or if Homer were to say "D'oh!" to complain about big things in life rather than hurting his thumb or whatever, they would become less of the endearing American institutions they are IMO.

I would not say that Charlie Brown is untouched by failure. He does descend to the depths of despair. But some how rises from it to try (and fail) again. This trope is seen best with Lucy pulling away the football every time he goes to kick it. Even though he knows he's failed every time, he talks himself into this time being different.

This does not contradict your overriding point, just adding nuance to the claim he is "simply not touched by failure".