Any fat, plump, chubby, chunky, or portley female that has an overflowing midsection or belly that protrudes over their waistline therefore resembling the top of a muffin that spills and rises over the base and wrapper.
Sarah is a muffin topper because her gut is showing and spilling out over her belt. Sarah was caught muffin topping because the bottom of her shirt didn't properly cover her fat belly today.
The jerk who only takes the top part from the muffins in the breakroom each morning, resulting in the proliferation of the less desirable "bottoms", which then become dry and inedible over the course of the day.
May you be hoist by your own petard someday, muffin topper