If you think about what Chopper actually did during Punk Hazard, it is surprising not that much.
Although he discovered the drug that was making the Children addicted, it was Law that took the poison out of them.
Although he wanted to save Mocha after she eat all the candy to protect the others, it was Sanji who lead the Marines and saved Mocha and the other Kids.
Even saving the people who were "frozen" by the gas weren't helped by Chopper. For how much Chopper was front and center during the Arc, he didn't really do much.
Although he discovered the drug that was making the Children addicted, it was Law that took the poison out of them.
Although he wanted to save Mocha after she eat all the candy to protect the others, it was Sanji who lead the Marines and saved Mocha and the other Kids.
Even saving the people who were "frozen" by the gas weren't helped by Chopper. For how much Chopper was front and center during the Arc, he didn't really do much.

