。
She's bad at sports.
Literal
She [topic-は] sports [subject-が] weak-at is.
苦手 is a na-adjective expressing personal incapacity or unease — 'bad at, not good with, find difficult'. Gentler than 嫌い ('dislike') because it frames the issue as one's own limitation rather than as rejection of the thing itself. The thing one is bad at takes が, not を — the canonical XはYが+adjective double-particle pattern that Japanese uses for skill, preference, and ability statements. Useful socially: it's a polite, self-deprecating way to bow out of activities.