When do you think Harry started wearing eyeliner? Or is it a pirate right of passage to have an on point eyeliner game except he doesnt always know what he's doing?
Omg maybe when Uma and Harry were younger, she stole some makeup from whoever and decided to test it out. So, she used Harry as her guinea pig along with herself.
She did a terrible job on Harry, but Harry was like “Cool! I look like I have two black eyes!” and thus Harry started using eyeliner on himself. Even if he never really learned how to use it properly it suits him.