。
She used to be pretentious.
Literal
She [topic-は] long-ago acting-pretentious [used to-ものだった].
気取る (きどる) means 'to put on airs' or 'to act pretentiously' — from 気 (spirit/mood) + 取る (to take), literally 'to adopt an air.' ~ていたものだった uses the ~たものだ pattern for nostalgic recollection of past habits: 'she used to be pretentious (looking back).' The ものだ adds a reflective, reminiscing quality that simple past tense lacks.