。
She does nothing but things that work against her own interest.
Literal
She [topic-は] herself [for-に] disadvantageous things [only-ばかり] is-doing.
損 ('loss, disadvantage') is used here as a noun in the attributive な-adjective form — 損な事 ('a thing of loss, a foolish-for-oneself thing'). 自分に損な ('disadvantageous to oneself') marks the target of the loss with に. ~ばかりしている ('does nothing but X, always doing X') uses ばかり ('only, nothing but') to emphasize that this is exclusively the kind of thing she does. The exasperation is built in: someone who keeps shooting themselves in the foot.