Mordekaiser's recent rework gave him access to good AD ratios on both his Q and his E. Despite building AD, most of your damage will still be AP. Because of this, it is difficult to itemize against Mordekaiser's mixed damage. Mordekaiser's Q in particular does crazy amounts of magic damage despite scaling off of AP and AD. His new E got a big base damage nerf, but still does solid AOE damage even when scaling with only AD. AD itemization also offers the benefits of having basic attacks that can pack a punch.

Because Mordekaiser is an extremely immobile and slow guy. Going full AD or AP will leave you extremely vulnerable to be bursted down in less than a second, leaving you no time to dish out any of your damage - despite how much higher it will be. Going tanky allows Mordekaiser to survive longer and actually get close to his opponents so he can do more damage over a longer period of survivability. Additionally, Mordekaiser's new passive shield scales off of max health - so it gets very big.

Your new Q does a LOT of damage. However, you need to attack 2 times after activating it in order for its last hit to reach its maximum damage potential. Getting this last Q proc to hit your opponent is your goal. If they are slow and have low ranges, this wont be too much of a problem. But if you find that they are running away every time after hitting them twice, try attacking minions twice before landing your third hit on them. Poke whenever you can with your E, and dodge/avoid any damage that you can. If they try to go all in on you when you're level 6, use your ultimate as a defensive healing tool rather than an offensive one. If you can consistently hit them with your third Q, then you will have likely won the lane. Hope this helps.