Yo Mama So Dumb and Poor

This is a classic "yo mama" joke, which means that it uses over-the-top ridiculousness to make fun of someone's mother, implying she's really dumb or poor. The first part is funny because it implies that she's so dumb she'd sell a car to buy the gas that goes in it. The second is funny because of how poor she is, even ducks feel bad for her.