It makes me wonder if we're supposed to think that Usopp - with his safe, peaceful and relatively not-too-bad background - is a lot more shallow than the other Strawhats and particularly than Luffy-the-protagonist-who's-suffered-so-much-more.

Nah, I don't think that's the case at all. I wish I had something more than a strong feeling with which to say this but that's all I have.