。
Pretending to be my mother wasn't difficult.
Literal
Mother [gen-の] pretense [obj-を] do [nominalizer-こと] [topic-は] wasn't-difficult.
振りをする is the fixed idiom for 'to pretend to be X' — literally 'do a pretense of X.' The larger structure ~することは難しい uses こと to nominalize the verb phrase into a subject. Notice that nominalization with こと (vs. の) tends to feel slightly more abstract or formal — の is the go-to in spoken language, こと for more generalizing or reflective statements.