I want to make a Topsy Turvy and was wondering can you use buttercream or is it best with fondant? I have never used MMF before so I was thinking of maybe just BC....I am scared I won't be able to smooth the MMF.

I've only done it with fondant, but I know that there are people who can do them in all buttercream. I think the trick is to refrigerate your cakes until the icing is very firm and then you can pick them up and place them without doing too much damage to the icing. It helps if you use a butter based buttercream - it gets harder and thaws less quickly than an all-shortening buttercream (IMO).