Please be more specific with your example of Kaido using his strongest attack on Luffy. Are you referring to the Ragnarok attack or something?
As for Pica vs Zolo, you have to understand that Pica vs Zolo wasn't a traditional fight. Pica & Zolo's goals weren't simply to defeat each other for the sake of it.
Pica's goal wasn't to fight Zolo, it was to destroy the Plateau and kill everyone on it, preventing them from reaching Doflamingo.
Zolo's goal wasn't to fight Pica, it was to save everyone on the Plateau, including Ussop, Viola and King Riku.
Pica, for 10+ chapters on end if memory serves correct, was outsmarting, outmaneuvering and wasting Zolo's time throughout the entirety of the fight.
Pica even emerged early on in the fight, giving Zolo an opportunity to no-diff
If Zolo was able to no-diff him he would have done so here, but no that's obviously not what happened. Zolo wasn't mobile or fast enough to catch him hence the fight dragging on for 10+ chapters with neither making any decisive move, Pica actually having the advantage nearly entire time.
Even though Pica didn't do any real damage to Zolo, it was a still a mid-diff because Zolo had to change the way he fights and actually strategize and figure out his opponent's ability to defeat him. It's part of the way the fight was structured that Zolo enlisted help from Orlumbus to defeat Pica, because Zolo himself wasn't mobile enough to reach Pica. He needed to find another way to overcome his mobility weakness because Pica was the one controlling the fight.
So in essence, Zolo was:
- Chased around, cornered and outsmarted by Pica for 10+ chapters on end
- Had to use brains over bronze because Pica wasn't a typical fighter
- Even when he managed to figure it out, had to use his 2 strongest attacks (barring Asura) to win
Conclusion: Mid-Diff