Puffy Pad wasn’t sure what it meant when her Cutie Mark turned out to be a diaper, but it didn’t seem to matter what it meant, because it lead to ridicule from her peers anyway.
 
Eventually, she snapped and decided to just embrace it. She began wearing and using diapers and dressing and acting like a baby.
 
After some time, though, she realized that nobody could laugh at her if they were in the same situation as she was, so she decided that she would use her unicorn magic to turn all the ponies of Equestria into her diapered baby slaves.