I've made several Barbie cakes - I'm hopeless at cake decorating and these were easy and looked OK.
I have what is known in NZ as a "Dolly Varden" cake tin (rounded cone shape) which is perfect for the skirt. I use a cake recipe that would usually go in a 23cmish tin, it takes longer to cook in the shaped tin and I put tinfoil over the top for the last 15 mins.
I buy the 'ballerina' style Barbies - i.e. Snow White or Swan Lake, as these come dressed in short sticky-out ballet dresses, which cover the messy bit at the top where Barbie's legs are stuck into cake.
I wrap Barbie's legs in clingfilm, and also cover her hair to keep it out of the way when icing. Cut an inverted cone out of the top of the cake and shove her in.
I just use butter icing, coloured bright pink, slap it on and stick on lollies/icing flowers etc.
I remove the Barbie before cutting the cake, remove the clingfilm and present it to birthday girl.