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She's not only pretty, she's also smart.
Literal
She [topic-は] pretty [not-only-だけでなく] head [also-も] is-good.
~だけでなく is the conversational 'not only X, but also Y' pattern, the everyday counterpart to the more written ~ばかりでなく. Notice the も ('also') in 頭もよい — it marks 頭がいい as the additional thing being predicated, beyond the established きれい. The pattern X だけでなく Y も... is the standard frame: the second item is marked with も to emphasize the cumulative reading. 頭がよい ('smart') and 頭がいい mean the same thing — よい is the slightly more formal or written variant of いい.