She's beautiful with her mask on but when she takes it off she's not that pretty.
"マスク美女だが外すとそうでもない"
- She's beautiful with her mask on but when she takes it off she's not that pretty.
- She looks pretty with her mask on but when she takes it off she's nothing much.
うちの妹はマスク美女ですが、マスクを外すとそうでもない。
- My younger sister is beautiful with her mask on but when she takes it off she's not that pretty.
- My younger sister looks pretty with her mask on but when she takes it off she's nothing much.
She's a "mask beauty" but she isn't so attractive without it.
My sister is a "mask beauty" but she isn't so attractive without it.
という表現が考えられます。attractive は「魅力的な」という意味で、容姿の美しさを表します。
「マスクを外すとそうでもない」という部分は、
she looks different without her mask.
（マスクなしだと違う顔に見える。）
she looks different than you'd expect without her mask.
（マスクなしだと期待していたのと違う顔に見える。）
のように表現することもできます。
アメリカ合衆国 
