Morde is actually considered quite underpowered at the moment and has pretty much never seen competitive play and probably never will due to the problems into his kit:
- He has an extremely weak 1-5 because all his spells cost health. He cannot reasonably trade with anyone.
- He has no mobility, no gap closer or escape. He has no way to survive ganks in lane.
- He is forced to push his lane if he wants to harass or trade, making him very open to ganks. Coupled with his lack of escape, he's extremely easy to shut down
- He has no CC or utility. He is pure damage. This is why he does so much damage, because that's all he does.
- He has no sustain outside of his ultimate, making Hextech Revolver a required item on him and limitting his laning ability severely pre-revolver.
If his E + R is killing you, it sounds like 1) he's already fed or 2) you have no health or magic resist built. Either case, it is your fault - not Morde being overpowered.
No champion is balanced around Twisted Treeline or Dominion, so I won't bother addressing those points. All balance is done with 5v5 in mind.