。
She was only pretending to be asleep.
Literal
She [topic-は] sleeping pretense [object-を] was-doing only was.
眠っている振りをしていただけだった — だけ ('only') marks that the sleeping was merely pretense, nothing more. ~ふりをする ('to pretend to') attaches to the ている form for the kind of pretense that involves continuous appearance, like fake sleep. だった confirms the past tense.