Wally is based upon a brilliant engineer who was denied an incentive package to leave the company because he was deemed too valuable. On that day he decided he would become as useless as possible. In a twisted example of corporate thinking, the company planned on getting rid of deadwood by offering packages to employees they did not value. They effectively rewarded the incompetent at the expense of the capable and competent.
The message was not lost on Wally. He is a genius.
Let's remember the difference again: Wally is not lazy, he is unproductive. He will go out of his way, sometimes even putting a substantial amount of effort (like climbing on a vent duct), to be as useless as possible.