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".