Mord is great but his early game is weak. He loses top to almost all bruisers levels 1-3. He can't generate enough shield in a burst if they gap close you, and he can't spam to keep it up due to HP costs. Another big weakness is that all his skills are AoE, meaning you can't harass without pushing your lane and making yourself vulnerable to an eager jungler.
Late game he tanks hard while dishing out respectable damage if he isn't chain CCd and his ult can turn a losing game into a winning game. Look at what all his ult actually does.
Let's say Mord has a modest 200 AP (Full build will be considerably higher)
42% HP Damage (half upfront)
Mord Heals 42% of their HP (half upfront)
Mord gains 14.7% of their HP in shield (half upfront)
That alone is a 98.7% effective lifeswing (half upfront)
If you secure the kill Mord gains a Ghost copy that deals that auto-attacks. This ghost benefits from their Armor penetration, and applies on hit effects including items like Frozen Mallet and Black Cleaver, and has all their passive and on-hit effects (Evelyn ghost will stealth, Ashe's Q if it is toggled on when she dies, Jarvan's On-Hit passive etc). The only passive on-hit effect I have seen his ghost not apply is Jax's ult, likely because Ghost Jax can't receive buffs.
Ghost Olaf? Darius? Insane power.
But wait, there's more!
Many people forget morde also gains 20% of the characters AD and AP.
Bonus AD scales into your Q which is magic damage and affected by your magic pen.
Bonus AP scales into all your skills of course.
So even if you target a fed AP carry who's ghost is relatively weak, the bonus AP can still give you a huge power buff.
Bonus! Mord ghost can cap altars! If you both stand on it, it will increase as though there were two champs on it, or if you are low on life you can cap the altar with the ghost from the safety of the bush.
All these factors together make Morde's ult one of the strongest in the game, imo.
Most importantly, as I mentioned at the beginning, Morde's ult can turn a losing game into a winning one. It directly scales with your opponent's HP, AD, and AP. Even if you are behind you can turn your opponent's advantage against them. If you're in a position where you can't focus down a single champ, well I guess you can't win them all.