Perhaps it's there to help her duel tanks (which if you look at a lot of top laners, there's a lot of things in there to help duel). A tank doesn't dish out a lot of damage so having part of her damage output be dependant on what here enemy dishes out makes it really bad against tanks.
And as far as other examples, we got camille true damage Q, Fiora %max health true damage passive, pretty much every tank (ornn, sion, poppy, gragas, maokai, galio) all having % max health damage sources in their kits, darius true damage (and innate armour pen), garen having natural armour shred, jax having hybrid damage (which makes him actually impossible to remove as a damage threat), bruisers like wukong and jarvan having % armour shreds. Even Yorick was given % health damage on his E (Yeah you probably forgot that one guy didn't you).
There are ones that don't have any tools to beat up tanks, like pantheon or kennen or jayce or kayle but they fill their own specific niches (early game bullying/range) and still have ways to scale offensively to beat up tanks (lethality builds)