I don't think of the zombies as Moria's strength level. He didn't use it when he was a famous pirate who got his Shichibukai status.
The biggest error in Moria's strategy was to give the combat out of his hands. When facing low-tier enemies, its ok.
Like most fallen villians in OP Moria might learn from his wrong strategy.
And don't forget: Luffy never fought a zombie! He did one blow to Oz, thats it. Luffy was only fighting Moria, and without the Nightmare Luffy powerup, the strawhats would have lost.
All strong techniques we saw from Moria weren't related to Zombies:
- Shadows Asgard
- Kage Kakumei
- Kagemusha
- Brick Bats
- Black cage
- Doppelman
I don't see Moria as a brawler, but he alone, not his zombies, is one of the Shichibukai. His strenght lies in his brains and his evil tactics. Personally, i see the Shichibukai as a balanced party of Pirates. Even Crocodile was not to be taken lightly! I don't think you can think in tiers… for the World Government choses them from the fear and respect they emit!
He has a 20+ feet body, he can hold an adult in his hands, he can mess with his enemies bodies, he can defend and attack with his own Doppelganger, he can dissolve and switch position with his Doppelganger (logia like).... Most people see a super-destructive badass Bartholomew Kuma as a lot harder to take out than a Gecko Moria... but he lurked in the Florian Triangle for 10 years and is still a feared member of the Shichibukai!
All his zombie affection lead him on the wrong path of lazyness... now, without them and a crushing defeat in his mind, we will see a new Gecko Moria who might fight more for himself with less arrogance and who upgrades his abilities.
(The DF abilities can be mastered in many ways… don't forget that, his path was hardly everything a shadow-man can do!)