。
She wasn't foolish enough to ask such a stupid question.
Literal
She [topic-は] that-kind-of foolish question [object-を] ask [to-the-extent-of-ほど] foolish was-not.
[verb-clause]+ほど+[adjective] gives 'to the extent of being X / X enough to do Y.' Here そんなばかげた質問をするほど ('to the extent of asking such a stupid question') sets the bar; 愚かではなかった ('was not foolish') falls below it: 'was not foolish (to that degree).' ばかげた is the past-attributive form of ばかげる ('to be silly, look foolish'), used adjectivally — 'silly, foolish, ridiculous.' 愚か (oroka) is more formal/literary than the everyday ばか, with a moralizing or evaluative flavor. The two synonyms together — ばかげた and 愚か — show Japanese's stylistic range from casual to formal within a single sentence.