So Morde's ult is supposed to steal 10% of the enemy's attack speed. However, it's been bugged and instead makes the enemy lose attack speed equal to 10% of YOUR attack speed. This makes your ult only good if you're building attack speed, which Mordekaiser doesn't do anyways.
This also explains why your ult does nothing to ADCs who stack attack speed.
Knowing how slowly Morde bugs have been fixed in this past, I'm hoping new Morde can get quicker bugfixes.
In the pictures, the dummy goes from 0.66 -> 0.38 attack speed with a full attack speed Morde build. Without attack speed, it goes from 0.66 -> 0.57.