。
She's by no means a fool.
Literal
She [topic-は] never fool is-not.
馬鹿 ('fool, idiot') is one of Japanese's most-used insulting nouns — the kanji literally write out 'horse-deer,' a folk etymology often traced to a Chinese story about a courtier who declared a deer was a horse to test loyalty. Tone matters enormously: among friends 馬鹿だなあ can be affectionate teasing, but used seriously it stings. The 決して~ない frame here functions as defense or recategorization: she's been called or perceived as foolish, and the speaker pushes back firmly.