You don't get to be High King of Fillory without some brains to match your magical brawn. Margo found a way to step out of Eliot's shadow as ruler at Castle Whitespire, even if part of her strategy was inadvertently cavort with a talking Bear. Margo is very adept at reading people, and possesses immense "street smarts." She knows when she's being lied to, and knows how to keep people in line. Not to mention, you have to harbor a modicum of intelligence to be a magician, since so much math is involved when casting spells. Ick. Math.