Some aren’t. Chemists have a sense of humor like everyone else. I think.
Most chemists I know have a lot of fun doing chemistry, and they got into chemistry because they were having fun. But most people take their work seriously, especially if it can change the world and improve people’s lives.
If your job was to create new molecules to cure a disease, or to understand how human activities are affecting the environment, or how to clean up nuclear waste, you would want to do a good job. But if you were a comedian or a circus clown, you would still want to do a good job. Most people take their jobs seriously.
Chemists must also be very careful when they work, so as not to injure themselves or others, and to make sure that the results they get are correct. In this regard they are similar to firemen and accountants, soldiers and rocket scientists, or doctors and engineers. All of those people are serious about their work too.