。
She's sharp.
Literal
She [topic-は] head [subject-が] cuts.
頭がキレる is a colloquial expression meaning 'to be sharp-witted / quick-thinking.' キレる (written in katakana here for emphasis) derives from 切れる ('to cut'), and the metaphor is the same as English 'sharp.' Writing it in katakana gives it a punchy, slangy feel — the kanji form 切れる would read more neutrally. This is distinct from the more negative キレる meaning 'to snap / lose one's temper.'