。
She's tired from overworking.
Literal
She [topic-は] work-too-much [cause-で] is-tired.
働きすぎ nominalizes the ~すぎる pattern — 'overworking' as a noun. The particle で marks it as the cause of 疲れている ('being tired'). This causal で + result structure is a clean, everyday pattern: [cause]で[state] — '(she is) tired because of overworking.'