I’d say the metaphor is pretty clear. You’re right. The sole sentence of “What is my purpose'” from the little robot already reflects the essence of the being. When Rick first says “Pass the butter””, the machine is fine with it. But then, when it asks again, it finally realizes its own mediocrity and dullness (because its whole purpose is only to pass butter), which is why Rick says “Welcome to the club, pal”, as in, welcome to my world, where humans are mediocre or as in, there is no deeper meaning to the question of one’s purpose. What it is left is the own absurdity of the world, where Rick and the robot live in. Of course people can have different answers, so one can therefore choose to take it lightly or seriously. Pretty interesting, huh? In a way, I’d say the reason why Rick and Morty works so well is because not only does the comedy work, but it is partly because of the laughs that come from intelligent shit underneath the surface.