。
She found the joke amusing.
Literal
She [topic-は] that joke [object-を] interesting-treated-as.
~がる is the auxiliary suffix that marks another person's expressed emotion or apparent feeling — Japanese requires this for third-person emotions because internal states aren't directly observable. From おもしろい ('interesting, funny'), おもしろがる means 'to seem amused, find interesting, treat as fun.' Note the particle shift: the bare adjective takes が (冗談がおもしろい), but the がる-form takes を (冗談をおもしろがる) — a regular pattern across the family (寂しい→寂しがる, 怖い→怖がる, 嫌い→嫌がる).