Doesn't matter if you let him farm, he is quite viable unless he gets zoned out the ass (like 2 levels below his lane). There is a reason he is banned almost continuously at most elo (my roommate is in 1900 [Champion Mage], one of his top bans, and the first ban in every single game he plays is morde). Morde can farm like a boss, can ignore ganks because of his shield (lol 2 people on me? Walk back to tower, regen health, start pushing again) and can win team fights with his ult (Ults your carry, hes legendary, you just got aced by Mordekaiser). You don't build morde as a tanky dps or a tank, you build him as a AP nuke. With 25% spell vamp, mordes ult makes him nigh unkillable if used on a beefy champ, and even if used on a carry, the rest of his abilities giving him health make fighting him in anything but a 3+v1 an idiotic idea.Yeah but, who is ACTUALLY stupid enough to let him farm? :P
morde is just not a viable option tbh. there are way better tanks/tanky dps out there.