I think it has more to do with the fact that Karthus is generally safer top from ganks than Mord, while also getting huge benefit from the "ignore everything and farm creeps all day" playstyle that's common top, while also not truly being as isolated as other tops because of his ult. (he's also got free mana returns on creep kills and doesn't, strictly speaking, need blue to perpetually remain in lane)
Mordekaiser, meanwhile, is the godking of wraith farming, and is much stronger mid than top.