。
She looks cute when she wears that outfit.
Literal
She [topic-は] that clothes [object-を] wear [when-と] cute appears.
~と here is the conditional 'when/whenever' — the natural-consequence と, signaling that wearing the outfit reliably produces the cute appearance. ~く見える ('looks/appears X') uses the adverbial form of an i-adjective with 見える ('appears'); the same pattern underlies 大きく見える ('looks big') and 若く見える ('looks young'). 服 covers any item of clothing, but in singular generic use often translates as 'outfit' or 'dress' depending on context. The whole sentence has the rhythm of a casual compliment.