Make the cupcakes following the directions on the box but only filling them about 2/3 of the way full. You don't want too much of a dome on them.
Color the frosting to your desired shade of green and set aside.
To make the lawn mowers
While the cupcakes are cooling melt the chocolate chips in the microwave for 30 seconds and stir. Continue heating in 10-second increments, stirring after each one, until completely melted. Pour chocolate into a disposable piping bag fitted with a Wilton #1 round tip.
On a sheet of wax or parchment paper pipe the handles for your lawn mowers by making a rounded rectangle about 1 1/2" high with a crossbar. Make more than you think you need in case some of them break. Put in the freezer for 15 minutes until solid.
While the handles set unwrap the Hershey's Nuggets and attach the M&M wheels using the melted chocolate. Freeze for 15 minutes.
Once the body and handles of the mowers have set remove them from the freezer and attach the handles to the body of the mower with melted chocolate. Do this quickly because the chocolate will harden fast on the frozen pieces!
Put the assembled lawn mowers back in the freezer for another 5 minutes.
Add your green frosting to a piping bag fitted with a Wilton grass piping tip. Or you can make your own grass tip by folding over the untrimmed tip of the piping bag, securing it with tape, and poking large holes in it with a toothpick.
Grass up your cupcakes with frosting!
Top with chilled, assembled lawn mowers just before serving.