Morde has a lot to do with metal.
Morde's passive is him doing damage with his metal abilities to strengthen his armored exterior with their life force.
Morde's Q is him smacking the person with a gigantic, spiked, metal mace.
Morde's W has literal metal shards circling him.
Morde's E super heats the floor with soul energy and turning it into metal.
Morde's R uses the power of Metal to rip the soul from some one.
Three of his abilities, passive, Q, and W, all are DIRECTLY related to Metal with his passive being his Suit of Metal, his Q being his Metal Mace, and his W having actual Metal flying around him and his partner. To say his kit isn't Metal related is like saying Heimer's kit isn't Turret based because only two of his abilities spawn a turret or that Ahri isn't the Nine-Tailed Fox because her abilities have nothing to do with her abilities (Her abilities all seem to be focused around that orb after all) or Bard isn't a Wandering Caretaker because his only actual ability to directly take care of his teammates is his W, the healing shrine, or Kalista isn't the Spear of Vengeance because she doesn't get any boost for taking vengeance on anyone.
Ultimately, titles are decided by Lore and there's nothing in his current lore or in the new lore that says Mordekaiser is the 'Lord of Death'. His current lore has him grow strength when around other's pain and sorrow and his new lore states he enslaves souls. I mean, if we were to keep this true to lore like the rest, he's the Lord of the Shadow Isles or Lord of Anguish or Iron Slaver (Since he's metal and he enslaves souls) or something... I mean, his kit is metal-based both Musically and Literally.