。
She has a round face.
Literal
She [topic-は] round face [object-を] is-doing.
~をしている is the standard descriptive pattern in Japanese for inherent body features and appearance — 青い目をしている ('have blue eyes'), 長い足をしている ('have long legs'). Literal 'is doing' makes no sense in English; Japanese uses する here in a 'have/possess as a state' sense, with ~ている locking the action into a steady, ongoing reality. The advantage over the plain noun-predicate 丸顔だ is flexibility: you can stack modifiers — 真ん丸い顔 ('perfectly round face'), 大きな丸い顔 ('big round face') — which the fixed compound 丸顔 resists.