To truly understand the character of George Costanza, you have to understand one thing:
Just because you have no morals, doesn't mean you are the Übermensch.
George has no morals. He lies when necessary. He cheats when it's in his best interest. He steals to further his goals. He uses his friends as stepping stones towards whatever he desires. He does whatever it takes to get whatever he wants, regardless of the consequences.
Still, he fails in most of his missions. That is where the humour in George Costanza manifests itself. The audience enjoys watching him fail time and again with his nefarious schemes. Parking in a handicap spot results in a destroyed car and servitude to his father as payment. Taking an unpurchased book into the washroom eventually leads to an arrest for shoplifting.
But if we continually watched someone fail at EVERY endeavour, the character would grow boring. This is the other part of the laughs in George Costanza. Even when he is bungling and bumbling, somehow, someway, he manages to win once and a while. While perpetuating the lie that he is a marine biologist, he is somehow able to become the hero by rescuing a whale from certain death. When he is at his lowest point and decides that everything he does is wrong, he somehow ends up getting his dream job and working for the New York Yankees.
George Costanza is one of the funniest characters in sitcom history because he represents everything wrong with us, and pays the price for his actions, but still gives us hope because he sometimes gets away with it. He's like the anti-Andy Griffith.