。
She's really good at imitating Mickey Mouse.
Literal
[topic-は] Mickey-Mouse [genitive-の] imitation [subject-が] very good.
上手だ ('skilled at, good at') uses が rather than を for the thing one is good at — like potentials, 好き, 嫌い, わかる, and a handful of other 'predicates of internal state.' Beginners initially expect を for 'object of skill' but the pattern is consistently が. 真似 ('imitation, mimicry') as a noun pairs with the verb 真似する ('imitate') — here the noun form is used since 上手 takes a noun complement. Mickey Mouse (ミッキーマウス) is a globally recognized Disney character, and Tokyo Disneyland (東京ディズニーランド) is one of Japan's biggest tourist destinations.