They're not making fun of crippled marines, is the point. The fact that he is in the joke doesn't mean he is being made fun of, he could just as easily have lost his hands in a logging accident or a machining accident. It's just backstory to set up the joke.
His helmet was stifling, it narrowed his vision. And he must see far. His shield was heavy. It threw him off balance. And his target is far away.