It might depend on the exact look you want.
Many people just use regular white clay (esp.the very soft chalky "original" white Sculpey that comes in a box) ....or they might use white clay mixed with translucent, or white clay thinned with various things (Softener-Diluent, liquid clay, veg oil, etc.).
But the shape
you want your whipped stuff to be could make a difference too. Some of those above could just be put on already smooth, or be smoothed on with a spatula (on cupcakes, e.g.), or shaped and or textured slightly (like on a cake).
If you want to have a fancy spiraled dollop on top though, you could either roll a clay ball into a tapered log, then arrange it. Or you could extrude it through a clay gun or through an icing tip into that shape (if thin enough it should thin out at the top when you pull upward.
For extruding, the clay should be softened, and very soft if you want it to droop at bit, or you could even make a "dough" of white mica powder (Pearl Ex, etc.) or microfine glitter, or maybe even chalk powder, cornstarch, baking soda, or who knows what else, mixed with a bit of liquid clay (or translucent clay?), then extrude it in a dollop shape.
For tiny amounts of clay like these, you could just push the clay out of a tip with a finger .
(There's more info on using icing tips, and clay guns, on this page if you want to take a look, including various other mediums you could use: http://www.glassattic.com/polymer/claygun.htm
You could just use white acrylic paint, and saatin or gloss glaze too, if you wanted to add a thin icing rather than whipped.
You might want to check on this page though to see if there are any suggestions, and also to look at other frostings
, etc., that clayers have made on their mini-foods:http://www.glassattic.com/polymer/miniatures.htm
GlassAttic....polymer clay "encyclopedia" http://glassattic.com/polymer/contents.htm
little bit'o photosharing: http://pg.photos.yahoo.com/ph/dianeatglassattic/my_photos