As predicted Momo's main role is to counter the fall of Onigashima.
How in the world people thought that he was going to scar Kaido?
Actually I think that most of the people thought that Momo will scar Kaido only to belittle Zoro so that they could say "look, scarring Kaido is nothing special also Momo can do it".
That's an elité feat, Oda won't give it to everyone.