Is it just me? Or does Ornn do too much damage for a tank?

Ornn doesn't have reliable CC or sustain. So he can't win wars of attrition or set up for ganks nearly as well as other tanks can

Ornn's W->auto chunks but it deals % hp magic damage, so it can be mitigated

Other toplaners have sustain, good CC, conqueror, shields, klepto...

Ornn does damage because he needs the damage to survive lane and have presence in fights beyond his R

Not really if you understand his trade pattern. Champs like Mundo (axe spam) or Sion are much worse.

Ornn needs to breath on you or hit you with his pillar for him to do his %HP damage attack.

Most of his damage comes from ccing you. Although any tank will do massive damage if they take CONQ atm

all toplane tanks deal damage not just ornn

not really, he does too much mixed dmg for a tank sure but not just too much dmg. here's the thing with ornn, you can't just get hit by all his shit and expect to win. figure out how to dodge something, and figure out what builds work against tanks.