Morde was reworked on patch 5.16. world's is on patch 5.18 just 2 weeks later. ever since the end of season 1 the meta has been support and adc bot lane. The LCS teams even have a position named adc. Morde is the first viable non marksmen adc in the game. it's a pretty big change to take in.
And the reason why he is perma picked/banned in worlds is because a large part of his kit is gated off by teamwork and effective communication. In world's it's not the bot lanes job to secure dragon and it's not the jungler's job to secure dragon. It's the teams job to secure dragon. It's also the teams job to make sure morde gets to the right targets. In world's the teamwork and communication required to bring out the best of mordenkaiser commonplace. The strategies to deal with it however, do not exist yet.
I've lost to morde some and i've also won against morde. winning against morde as a botlane takes a drastically different tactics then what i would use against any conventional botlane. losing against morde merely requires me to play like i've always played. but i have noticed a few things.
1). the time limit placed on how long you can use dragon. If you hurry you cant take the mid and bot outer turret, and inner turret and mabye deal some damage to the inhib turret but that's as far as you can go with the time limit. if you don't go to a tower immediatly after dragon your wasting a large portion of morde's strength. The bad morde's don't use dragon ghost effectively or care about dragon control.
2). he's very tanky early level but doesn't do much damage. if he has a squishy support (rare i know) kill him first then kite morde around to kill him.
3). caitlyn can poke him out early, but ashe can kite him better come lategame. in fact during lategame outside a flash 3rd q ashe can make morde worthless just by keeping him pinned to the same place.
4). graves is a horrible to pick into mordenkaiser.
5). he likes to push his lane. i had limited succes with a blitz or thresh hooking them into turret range.
6). i haven't figured out how to counter morde with a tanky engage support yet, still working on it.