。
She found her son's foolish antics amusing.
Literal
She [topic-は] son [genitive-の] foolish acts [object-を] thought-amusing.
おもしろがる ('to find amusing / to enjoy') uses the productive ~がる suffix that turns adjectives or feelings into observable behavior verbs (面白い 'amusing' → 面白がる 'show amusement, find amusing'). Same pattern as 怖がる ('show fear'), 嬉しがる ('show joy'). ばかなまね uses the [na-adj]+な+まね pattern — まね ('imitation, act') with をする gives 'foolish behavior / silly antics.' The whole sentence describes a fond, indulgent attitude.