User blog comment:Kaido King of the Beasts/Chapter 881: Look at Me, I'm the Helmsman Now/@comment-24850639-20171005152544/@comment-24850639-20171006134929

You could also argue that he pinched his mouth so that the crew wouldn't hear him taking hits or his agony, or that the big goofy smile was to put on a front that everything was ok on his side. That would help explain Jinbe's solemn face while the rest of the crew rejoiced at the plan to meet up on Cacao. In either case, it was absolutely crucial that he hid his face from Katakuri.