I've never had a muffin top because I'm wise enough to buy jeans that aren't too low waisted. That's the problem - girls with too much belly wearing too low rise jeans. It also occurs on girls/women who are apple shaped, as they don't have any hips so have to pull their jeans in tight with a belt to stop them falling down - but then give themselves muffin tops.
My sister always has one and it drives me crazy, but then she's brick shaped and I'm pear shaped.
ETA; The girl in the pic would look so much slimmer, if she wore jeans that were higher waisted and a couple of sizes bigger. It's heartbreaking because she seems to have a tiny waist and curvy hips/bum - what a waste!!