You had a lot of hair cut off. Don't you miss your long hair?
Wow, you really had your hair cut short. Don't you wish you still had your long hair?
この場合、次のような言い方ができますよ。
ーYou had a lot of hair cut off. Don't you miss your long hair?
「かなり髪切ったね。もったいなくなかった？」
ーWow, you really had your hair cut short. Don't you wish you still had your long hair?
「わあ、かなり短く切ってもらったね。もったいなくなかった？」
ここでの「もったいない」はこのような言い方をするのが自然で良いかなと思います。
ご参考まで！
カナダ 
