what tip should i use?

Isn’t that fondant?

Yes, it’s ruffled fondant. Not sure about piping it to achieve the same look :-)

You won’t get the exact same look with buttercream but the ones I have seen they use Wilton tip or Ateco tip 104. Like this one: http://jessicaharriscakedesign.com/2013/02/buttercream-frills-or-ruffles-cake/ and I have seen one using tip 406 like this one: http://bubbleandsweet.blogspot.com/2012/05/how-to-make-ruffled-buttercream-rainbow.html