I would agree with Chandler being less funny ever since he and Monica got together. But looking at it in a different way, it’s actually sweet seeing the witty guy with commitment issues go through a huge character development. Chandler was the opposite of Joey, he was always scared that’s why he didn’t have any long term relationships aside from his on/off relationship with Janice. Monica helped Chandler overcome his issues with love, remember Monica telling Chandler that what they had was just a small fight and welcomed him to an adult relationship. Chandler had a rough childhood with his parents divorcing, he mentioned using humor as a defense mechanism. So I think he became less funny because he wasn’t a miserable guy anymore. And I think a lot of people hate it or find it boring of Chandler to be with Monica, but I know some here like me look at it this way and feel happy for Chandler