。
She looks happy.
Literal
She [topic-は] happily is-visible.
幸せに見える uses the bare adverbial 幸せに with the perception verb 見える ('look, appear'). Compare with the variant 幸せそうに見える, which adds the visual evidential そう: そう makes the appearance more inferential ('appears as if happy'), while plain 幸せに見える reads more directly as 'looks happy.' Japanese has multiple tools to dial how strongly an outward impression commits the speaker.