Takashi Tezuka gives a brief status update for the game including comments on difficulty.
http://www.nintendoworldreport.com/news/34628
Nintendo's Takashi Tezuka has briefly spoken about the status of the Wii U Yoshi game, Yarn Yoshi, in a recent interview.
His explanation of why the game was absent this week was that it is not in a state that Nintendo feels is ready to be shown. He also touched upon why Good-Feel was chosen as the developer, stating, "I looked at [Epic Yarn] and thought it was really beautiful. It seemed like it would be a good fit for Yoshi as well. We found that the Wii U would be able to make it a much more expressive game. It looked like it would be a lot of fun."
One other thing that was discussed was difficulty balancing within Nintendo's platforming titles. Tezuka noted that he would like to incorporate more of a challenge in the new Yoshi game if possible.