expression
wearing light clothes for style; dressing lightly to look fashionable
A set phrase describing someone who wears thin or insufficient clothing in cold weather out of vanity or a desire to appear stylish. Often used critically or humorously.
真冬にコートも着ないなんて、伊達の薄着もいいところだ。
Not even wearing a coat in the middle of winter—that's taking 'fashion over warmth' too far.
彼女は伊達
She was dressed lightly for the sake of fashion and was shivering in the cold.