I use a tip 12 with buttercream and just sort of randomly puff it up. Is that the kind you are referring to? I have a couple of examples in my gallery (Thomas the train and winnie the pooh)
using icing pipe with tip 12 and on a 45 degree angle use strong pressure and gently pull back , or you can just stop and smooth out the tip of the cloud with some powder sugar.
I also pipe them on with the #12 tip. then I take an icing spatula and "fluff" them up a bit. 
you can also do it in a back n forth motion to give it some texture
when i do clouds i use tip 12 or larger with a medium-thin consistency (allows the icing to have a more fluid "fluffy" look) and make "blobs" for a 3D look i build upon the other. to avoid the peeks before lifting your tip stop all pressure and with a swirling motion slide the tip off the cloud.
hth 
Quote by @%username% on %date%
%body%