What matters is not her ability but her personality.
Literal
Problem [copula-な] [nom-の] [topic-は], her [poss-の] ability [not-ではなくて] personality [copula-だ].
Uses 問題なのは (with explanatory の) and ではなくて (te-form of the negative copula) to set up a 'not X but Y' frame. The contrast between ability and personality is a recurring template for shifting the focus of judgment.