Or, more to the point, is Quinn nothing but a little self-obsessed snot with a small nose?
I'm not convinced she is. Let's look at the evidence, excluding "Quinn the Brain" for the moment:
She's capable of acting rationally, even with Daria. Their godfather-like negotiation when deciding not to tell their parents about the paintball game in "The Daria Hunter", for example.
She's capable of making sound decisions when it comes to boys. At the party in "The Invitation", Quinn refuses to go make out with Joey, Jeffy, or Jamie, since she "doesn't feel ready" for that. Nothing we've seen requires us to think that she's out givin' it up to every guy with a nice car. I'm not sure what to make of the fact that at the same party, Jane - - oh, never mind.
She's Daria's sister. The family is largely composed of intelligent people. Helen and Jake are, aside from their slight (or more than slight, in the case of Jake) maladjustments successful businesspeople. The aunts in "I Don't" seemed reasonably bright, too (especially Amy, of course). There's gotta be something going on behind that cute little nose.
BTW, in "Monster", when Quinn takes a hangar out of Daria's closet, it's an exact duplicate of the outfit she's already got on, with a green jacket and black skirt all on one hangar. But what seems odd to me is that there's other clothes in the closet. Gasp!
-Monty