What? Zoro didn't tell Luffy to become a asshole, he told Luffy to be more serious because he was joking around with Ceasar. Luffy isn't laughing around, he pissed off. Also its not only Tama, Yasu suffered, died, the people of Wano were starving to death and still offered Zoro food. Read the damn manga. This isn't out of character for Zoro, Zoro has always acted like this.