Humor does not equal goofy, at least not for me. YMMV.If you lived in France all your life, but jumped ship in the US to search for a better life, you'd still speak, talk walk and act like a Frenchman. You don't flip a switch and become a different person in a blink.
He's very goofy. Most of the humor in TFA comes from Finn. What movie were you watching?
And we still see the soldier come out, specifically in combat. The rest of the time he is trying to fake being that ordinary guy. But, it doesn't come across as perfectly natural.