。
Her musical talent was just so-so.
Literal
She [topic-は] music [of-の] talent [topic-は] so-so was.
まあまあ is a 擬態語 doubling that sits at the verbal heart of polite Japanese understatement — neither praise nor criticism, just 'middling, passable.' Used as an answer to '元気?' ('how are you?'), it means 'so-so'; used as a quantifier, 'about average.' The は on 才能 makes the talent a contrastive topic ('as for her musical talent specifically...'), implying she may have been good at other things.