Your mama is so fat, when her husband said, "Let's go to the Super Bowl," she bought a spoon.
Super Bowl Spoon
Explanation
Explain Bear
Okay, so basically, your mama's weight is being used as a punchline. The joke is that she misunderstood the phrase "Super Bowl" as a literal bowl, as in, a huge freakin' bowl. And, because she's a big gal, she thought she'd need a big freakin' spoon to eat from the bowl. It's funny 'cause you're not too bright, but this joke ain't exactly rocket science, bro.