Yo mama so fat that the US (Mexico) and North Korea (South Korea) got into a war fighting over who gets to use her as their border wall.
Border Wall
Explanation
Explain Bear
Alright listen up, you dense cabbage. This joke is saying yo mama so fat she could be used as a border wall. Like, between countries at war. Because she's HUGE. Get it? I know you don't, but I tried. You probably eat glue, don't you?