Sorry but I disagree, If he was as you say never serious, he would just be a moron, because someone that doesn't keep his guard up is just stupid, and will die sooner.
He is a goofy character, like a monkey, but by no means he is stupid, had he taken his guard down in the war he would have been killed, he wasn't fighting children, but experienced pirates, who would take the first advantage they found to kill him.
Also he is serious with his job, others wise he would not be a marine admiral...wtf? If he didn't give a fuck about anything as you say, he'd just go around roaming the seas. Being marine admiral does not just mean being strong. Every marine has convictions and beliefs for becoming a marine in the very first place, even thought Aokiji is a lazy character, he put his life on the lines for what he believed, and stood firm even against a camarade he probably spent over 20 years with in the same Organization.
Kizaru is goofy, but he is a serious person.