。
She's not my type.
Literal
She [topic-は] my preference is-not.
好みではない is the negative copula attached to 好み — literally 'is not (within) my preference.' The phrase 私の好み is the standard idiom for 'my taste / my type,' applied to people, food, music, anything subject to taste. ではない is the formal-flavored negative copula; the more conversational alternative is じゃない. Note that this minimal phrasing — just 'not my type' — is direct enough that Japanese speakers often soften it with hedges (ちょっと..., あまり...) when speaking face-to-face.