Moped drum brakes suck. You cant compare it to a cantilever or style brake because it is stopping in different ways. Also your walmart bike weighs like 20lbs. You moped weighs over 100lbs.
The brakes may have started to glaze over after years of use. Causing a slick surface. You can sand your brake pads to roughen them up. You can also file in some grooves which creates more stopping points along your brake pads which can help. Also clean up the drum area real good with some brake cleaner. They make aftermarket pads you can grab if what you have just wont work.
If you want really good stopping power. Purchase a replacement set of forks that have a disc brake setup.