A tank needs a reason to be targetted, or to be relevant to a team fight.
Mord's a poor tank if you just stack durability on him. It's the main reason I avoid Warmog builds in many cases, and on Mord in particular.
My pick for Mord is a Rylai -- it gives him a damage pump along with an HP boost, and sustained utility and soft CC that encourage the enemy team to "do something about the meat".
But regardless, a tank needs to be relevant, and then afterwards get enough durability so that they can take the hits after the enemy ascertains that the tank needs to die, or take some threatening damage, or needs to be CC'd, etc.
Mord is not very relevant until he gets damage, CC, or both. When he does, he needs some tank gear to take some focus fire afterwards.