。
She has an earnest, deeply serious nature.
Literal
She [topic-は] earnest [of-な] disposition is.
きまじめ (生真面目) intensifies まじめ ('serious, earnest') with the prefix 生 ('raw, pure, unalloyed') — giving 'thoroughly earnest, deeply sincere' with a faint connotation of being almost too serious for one's own good. The same 生 prefix shows up in 生意気 ('cheeky, raw-cheeky'), 生半可 ('half-hearted'), and 生兵法 ('amateur tactics'). 気質 ('disposition, temperament') is a Sino-Japanese noun for innate personality character. The closing である is the literary copula — equivalent to だ but feels written-formal, common in essays, narration, and formal writing. The whole sentence has a measured, character-sketch tone.