。
She is no fool.
Literal
She [topic-は] fool [is-not-ではない].
馬鹿 ('fool, idiot,' literally 'horse-deer') is one of the most common Japanese insults but also frequently used affectionately or self-deprecatingly. The ~ではない is the plain negative of だ ('to be'). Said earnestly, this defends her intelligence; said sarcastically, it can have the opposite implication.