Hi Tiia, Is it actually quite a challenge to create responsive landing pages (emails are easy) which are mobile optomised. As Breno noted - Design Studio built landing pages are absolute positioned and by definition not responsive. It isn't impossible to make an LP responsoive however - you just need to "break" the design studio for it to work.
I stumbled upon a great website by a person called Edrward Unthank who provided a nice set of templates and approach for creating responsive landing pages which helped me and should help you to! The approach uses the (which he nicely coined) "double div" approach!
http://edwardunthank.com/blog/marketo-responsive-method-marketo-double-div/
http://yesler.com/blog/marketing-automation/4-reasons-to-use-responsive-marketo-landing-pages-and-how-to-create-them#.U7UNuW2wV1J
Thanks
N