These are Oda's own words remember
Go back to one piece magazine volume six. Oda made comment that he refused to draw a cover story where Zoro messed up a Sword technique even as as joke. He ended up the comment by saying "Don't make light of Zoro" or something like that.
So its not that you can't make jokes involving Zoro, I mean Oda does that himself in the actual manga... It's that there are types of Jokes that don't work with Zoro to the point that not only did Oda himself refuse to make such jokes, he went out his way to write a message to the person who requested such a joke explaining why he refused to do it