When I do cupcakes, I use a basic buttercream 'frosting' (standard buttercream recipe) for my swirls rather than something labelled frosting, which I think have other ingredients to make them lighter and fluffier. I use royal icing sugar instead of regular as it gives a slightly firmer set and a satisfying form, then a very slight crunch when bitten. I make mine in my Magimix processor and this makes it quite fluffy. I tend not to add all the milk/liquid quantity a recipe suggests, preferring a slightly firmer buttercream that handles better. Although sweet (obviously!) buttercream is scrummier than glace

On a giant cupcake, I'd say that as long as you generously and closely cover the surface of the cake with your frosting decoration, you shouldn't need a base layer. WIth buttercream or frosting, you can even use a palette knife to slather it on then sculp it into a rustic swirl, no piping tools needed. You could have some fun with lots of little swirls or star-rosettes - dot different colour ones in an out of your base colour. That could look quite funky!
Are you using one of those giant cupcake moulds?