。
How smart she is!
Literal
She [topic-は] what mind [subject-が] good [explanatory-の] [I wonder/exclamation-でしょう].
頭がいい works here as a predicate (in the main clause), so the subject marker stays が — contrast with the noun-modifying use where が would convert to の. The exclamation is bare and direct: なんて + adjective phrase + のでしょう. Note the double-subject structure typical of body-part predicates: the topic 彼女は sets the framing, and 頭が picks out which part of her is being described as 'good.' Same logic appears in 象は鼻が長い ('elephants have long noses') — the textbook example of double-subject Japanese.