。
She's not much of a poet.
Literal
She [topic-は] great-of poet is-not.
大した~ない is the dismissive negative pattern: 大した ('considerable, significant') paired with a negative gives 'not much of a / nothing special.' In the affirmative, 大した~ rarely appears in modern Japanese; the construction is essentially fossilized in the negative. 大した詩人ではない ('not much of a poet'), 大した話ではない ('nothing special / not a big deal'). Here in polite ではありません.