Because Karin is her mother.
Sarada might have been born half blind so she might have needed glasses. Karin was helping in the birth and since Sasuke and Karin had *** while they were kids she needs those damn glasses.
It's understandable why she dresses like Karin (She gets her clothes from her).
But then why does she wear glasses like her also? Do either Sakura or Sasuke actually needed glasses?
You guys do know that you can need glasses without a parent needing them? Neither of my parents need glasses at all yet I have very poor eye sight.
Well bad eyesight can be hereditary but that doesn't necessarily mean that it's inherited directly from either parent. Things like hair/eye color, diabetes and even skin tone can be inherited from ones grandparents or great grandparents.
Also bad eyesight can be developed through playing games in the dark, reading in the dark, and watching TV nearsighted.