。
No wonder he's angry.
Literal
Belly [object-を] get-angry [nominalizer-の] [even-も] natural is [emphasis-な].
腹を立てるのも当たり前だ means 'getting angry is only natural.' 腹を立てる literally means 'to set one's belly upright' — an idiom for getting angry. 当たり前 means 'natural/obvious.' な at the end seeks agreement.