I don't particularly find Grey Poupon amazing in any special way. It's just memorable because of one piece of strikingly good advertising, the "Son Of Rolls" ad featuring older gentlemen passing some fake french mustard between two cars. It was genius because of the ear-worm "Pardon me, do you have any Grey Poupon" line parodied a million times.
My dad told a joke, frequently, when he saw either the mustard or the Rolls-Royce. He'd go "Pardon me, do you have any grey poop on your window?" Get it? It sounds like "grey poop on."
I'm not sure if this joke was his own creation or he copied it, and I'm not sure which would be worse. Still, just like the original ad sticks with most people, I can't see Grey Poupon without thinking of that lame joke and then, of course, thinking about my dad.