。
She looks very happy.
Literal
She [topic-は] very happy [as-に] appears.
に見える with an adjective or noun gives 'looks (like) X,' based on the speaker's direct observation rather than hearsay or guess. The closely related ~そう ('seems') attaches more impressionistically and feels slightly less anchored in concrete visual evidence than に見える, which carries a stronger 'I see this with my own eyes' flavor.