Depending on the lane you should either play super passiv or aggressiv. Becaseu your dont have mana and good sustain on w you can go on crazy traids which you will heal up again with ravanous hunter and your w.
Try to freeze and let the wave push into you, if you have a long lane to run it down the enmy has no chance.
Try to use your Tp early to lane to get boots, your stats are strong enough if you can stay on the enemy

Generall: Because it steals stats you can ult people just do make them lower hp and able to 1 shot with Q.
In 1v1: Use it when you think it wont allow them to still hide under tower.
In 2v1: Try to ult the Person you can 1 v 1, if you can 1 v 1 both ult the person that gives you better and more stats to be used against the other one. They mostly will think they can kill you after your R, safe your w for when you get out to get a massiv shield.
In Teamfights: Ult champs who step to far forward so their team can help them. Other good targets are champs like you that give you stats you can use good. For example: Swain, Midlane Nautilus, ap tanks in general.
In fight objectives you can also ult sm1 to eiher make the jungler not able to smit the drake or not able to hit a tower becasue they are out of range.
If you are not the carry of your team ult the greatest threat to your teams carry.
YOu dont always need to kill with it, sometimes its enough to buy time for your carries.

Mordekaisers midgame should be spend on the sidelanes.
Becuase of your Tp and good pushign potential added to your passiv and nashors tooth mordekaiser can suprisingly splitpush very good and even if many enemys come to kill you you can kill alot of them by your own which gives your team time for objectives.
This is between 1-3/4 items. Depending on your build.

In lategame morde falls off.
You can start teamfighting after you got 2 defensive items.
You wotn be able to 1 v 5 anymore which is why you shoudl use your ult with more thought put into it to take out an enemy for the 7 seconds and giving your team the free time it needs.