Very nice tutorial!!! They make cute flowers!!
I would think a large ruffle tip or a large rose tip would do the job. turn your bag on it's side and move your tip up and down, instead of side to side, and you turn the cupcake in the other hand. I would go backwards a little bit, every so often to create that whimsical uneven look.
I agree with ellavanilla. That is definitely frosting and not fondant/gumpaste although the tutorial linked is pretty :).
AYou could make these with a closed star tip. Just squeeze hard and it comes out like that! Try a 31, so you don't get too much icing on it when you squeeze.