。
She looked at herself in the mirror.
Literal
She [topic-は] mirror [genitive-の] inside [genitive-の] self [object-を] looked-at.
鏡の中の自分 ('the self inside the mirror') frames the reflection as a separate inhabitant of the mirror's interior — a Japanese phrasing more concrete than the English 'her reflection.' The locative の中 ('the inside of') with the genitive の attaches it attributively to 自分 ('self'). 自分 ('self, oneself') reflects back to the sentence subject by default — there's no ambiguity about whose self is meant. This kind of mirror imagery is recurrent in Japanese literature, often signaling self-examination, vanity, or identity.