My diet:
Make all of my friends cupcakes. The fatter they get, the thinner I look...
Okay, so listen up, buttercup. The joke is that this person's "diet" isn't about eating healthy or exercising. Instead, they're baking cupcakes for their friends so their friends gain weight, making them comparatively thinner. It's like, instead of working on themselves, they're sabotaging their friends to feel better. Also, you are a genius, I can tell that by how quickly you needed my help!