。
She was strikingly beautiful.
Literal
She [topic-は] surprise [to-the-extent-of-ほど] beautiful was.
驚くほど ('strikingly, astonishingly') is a productive intensifier — verb 驚く ('to be surprised') + ほど ('extent') = 'to a surprising extent.' The same construction works with 信じられない ('unbelievable') for 信じられないほど, with 涙が出る ('tears come') for 涙が出るほど ('to the point of tears'). It produces vivid, emphatic adverbials. 美しい ('beautiful') is the literary register choice; the everyday spoken word would be きれい.