You forgot something.
Assassins like Zed have mobility out the bum. Not to mention high AD ratios and an ult that turns him untargetable, allows him to shift around the fight and get out.
His victim then explodes like those delayed sword slash killings from action anime.
Is that bad? No, but what good is waveclear or burst if you die within a second after someone targets you?
Zed along with other assassins have the distinct ability to murder squishies no matter how fed. Their superior mobility to mages makes them a better threat becuase at least you can outrun a mage.
Is this bad? No, I slaughter Zeds when they lane agaisnt me. Has he been overnerfed? Probably. Preseason is giving him a new mechanic so he picks his target better, so we will see.
Also, why are you surprised that mages are the strongest midlaners when midlane is pretty much tailor made for mages?