If you want lime mousse on top, it's a good idea to start with it, as it needs a couple of hours in the refrigerator. But you can also just top with icing or a bit of coconut ?
Lime mousse is made like this:
Soak gelatin in cold water for 10 minutes. Boil lime juice with sugar, remove from heat and melt gelatin into it.
Whip the cream stiff, add lime zest. Mix the lime/sugar mixture with the yogurt and fold this mixture into the cream. Add food coloring if desired.
Refrigerate until use.
After a couple of hours (it should be set), you can put the mousse in a piping bag and use it on your cupcakes ?