。
She's easygoing.
Literal
She [topic-は] easygoing is.
のんき (呑気 / 暢気) is a na-adjective meaning 'easygoing, carefree, unhurried, optimistic in a relaxed way.' It often carries a faintly mixed connotation — admiringly when describing someone unflappable, slightly critically when implying someone is too laid-back to grasp the seriousness of a situation. Compare 楽天的 ('optimistic') for a more straightforwardly positive 'sees the bright side.' At three morphemes (彼女・は・のんき・だ), the sentence is about as compact as Japanese predication can get.