"So-so" would be 「まあまあ」 in Japanese.　But 「まあまあ」 on its own could sound a little abrupt. We would say 「まあまあですね」「まあまあです」「まあまあかな」「まあまあだよ」 and so on. - "Are you famous?"- "So-so," the singer said. would be 「あなたは有名なのですか」「まあまあですよ」とその歌手は言った。

If Japanese say "sousou", then it means "yeah right".