I thought it was funny too. I think I can clear this up or at least, try to. When he said under his breath, "Thank you, Beavis and Butthead!", I think it was meant to tell us that this guy thinks of himself as an egotistical snob. In his mind, he was probably thinking, "Out of all the secret codes in the World to open the richest bank vault in the World, this has to be the absolute worst secret code I've ever heard in the history of my life! I could've thought of a better, classier one than this!" And in keeping with the "high and mighty" character that he is, he said, "Thank you, Beavis and Butthead (meaning Mr. and Mrs. Rich)!" He was calling them that, I think, because he thought that the code they thought up had no-class and was not up to his "uppity" standards. That should tell us right there that this man has no sentimentality in his body, like they do! That's probably why they secretly never liked him! He's not a warm, sensitive, and caring person. He's a cold, obnoxious, and fortune-seeking *beep* Unlike, the Rich family, all he cares about is the all-mighty dollar. He taken care of their finances for years, so that might explain it! If it were the other way around, if things were done his way, he'd love the code he'd put in, while the Rich family might mockingly go, "Thank you, Napoleon Bonaparte!"
It sucked. It was too plain, not creative and in no way original. It seemed like a one-liner shot they threw in right at the last second, needing some comic relief in the situation. One sentence uttered from John and it ruined seasons and seasons of Night Court.
I agree. I like it because he clearly thinks the Rich family are spoiled brats who don't have a brain cell between them. By contrast he's presented as a guy who had to work and use his cunning to get rich (unlike the comic-book Van Dough who is also born-to-the-manor). Basically he was expressing his resentment over how stupid many people who were born rich are, and that's funny.
I was going down the list thinking about that line (mdilliner) and as long as richie rich is a movie, and even though the king of pop is dead, that line will never, ever get old.