The appearance is borderline scary, but they're actually quite nice.
Just by looking at them I think they're scary, but actually they're nice.
ご質問ありがとうございます。
この場合、「見た目が怖そうなひとほど、優しいよね」は英語で「The appearance is borderline scary, but they're actually quite nice.」になります。
また、別の言い方ですが、「Just by looking at them I think they're scary, but actually they're nice.」になります。
上記２つの英文で相手の怖さそうな事と実際に優しいな場面のことを伝えられます。
ご参考になれば幸いです。
アメリカ合衆国 
