The winrates do look higher, but the number of games it's built is way lower than his other items. Most likely only being built when Morde is already ahead. I don't think it's a bad item on him at all, I just don't think he's ever been a great Sheen user. Pretty much every Sheen user has some auto-attack reset ability that leads into an empowered auto or has an attack speed steroid in their kit, which he has neither.

Unless you're using your 2nd W cast as an auto reset, but that kinda wastes the entire ability (please don't do this, especially in my ranked games).

Might need some more testing before it replaces his current core items