Mordekaiser's kit and numbers would work and be viable if he did entirely Physical damage

A Juggernaut with unreliable itemization is a bad Juggernaut

Death's Dance is like a dream item for him but he can't use it at all currently

Black Cleaver is another item that in theory he would love but he has no Physical damage

Maw is another item he would synergize incredibly well with if he was more Juggernaut-ish like Illaoi and Darius

Honestly none of his problems are that his base stats are too low or anything like that, he just has no reliable itemization like every other Juggernaut. AP Champions with no Mana can't really build Bruiser, but if Mordekaiser dealt entirely Physical he would have a much easier time building in Jungle, Top, Mid, and Bot.