。
She's crazy.
Literal
She [topic-は] head [subject-が] strange is.
頭が変 literally means 'the head is strange' — a blunt, casual way to call someone crazy or weird. 変 is a na-adjective used in its plain form (変だ). This is direct and colloquial; more polite ways to express the same idea exist (おかしい, 変わっている), but 頭が変 has a raw, unfiltered quality.