That person has no common sense at all. His parents must not have done a good job raising him.
That person is so absurd. I bet it's because of the way his parents raised him.
この場合、次のような言い方ができますよ。
ーThat person has no common sense at all. His parents must not have done a good job raising him.
「あの人は全く常識がない。彼の両親は彼を育てることを上手くやらなかったに違いない」＝「あの人はすごく非常識だ。彼の両親の育て方が悪かったに違いない」
ーThat person is so absurd. I bet it's because of the way his parents raised him.
「あの人はとても常識を欠いて不条理だ。彼の両親の育て方のせいに違いない」＝「あの人はすごく非常識だ。彼の両親の育て方が悪かったに違いない」
すごく非常識なことは to have no common sense at all や to be so absurd などを使って表現できます。
ご参考まで！
カナダ 
