She refused to be bored chiefly because she wasn't boring.

The line turns “boredom” from a condition imposed by the world into a symptom of one’s own inner life. Zelda Fitzgerald suggests that people who are genuinely curious, imaginative, and self-directing can resist boredom because they bring interest with them; they generate their own stimulation rather than waiting to be entertained. The phrasing also carries a social edge: “refused” implies a deliberate stance against dullness and convention, while “chiefly” wryly undercuts any romantic notion of heroic resistance by attributing it to a simpler cause—she herself was not dull. It’s both a compliment to the subject and a critique of those who habitually complain of boredom.