。
She seems happy.
Literal
She [topic-は] happy-seemingly is.
幸せそうだ uses the bare visual evidential ~そう (with copula だ) — 'looks happy.' This is the simplest construction for an outward impression: no 見える, no 見える, just そう+だ. Compare 幸せそう (suggesting outward signs) with 幸せらしい (hearsay/inference from secondhand evidence) and 幸せみたい (casual, similar to ようだ). Each tunes differently: そう leans on visual cues, らしい on report, みたい on overall sense.