Nintendo has struggled with 3rd party support ever since the N64 and their choice to stick with cartridges. It has been getting better with the Wii, even though it opened the flood gates of shovelware. It should be better at least at first for the Wii U, and continue to be better as long as 1) Some gimicky cash in title genre doesn't give them get rick quick dreams like the party games did this last gen. 2) The Wii U consumers actually buy the "good" 3rd party games but showing them 'yeah we're here and we will buy it if it comes.'
The problem with the Wii was that Nintendo decided to release lighter titles that shouldn't of sold as well as they did. The third parties seen that and tried to cash in causing a flood of Me too party titles. (Take Two's Carnival games) The games that were released to test the water didn't sell well because either the ports were not handled well. (Rockstar's Pong) or ignored (Rockstar's Bully). Rockstar/Take Two Interactive is a good example of what went wrong with the Wii's third party support. And it seems Nintendo has corrected some things to try and intice at least better titles this time around from the bigger publishers as well as the smaller developers. Even to the point of one the of mobile developer saying that they made more money on the Wii's Wiiware channel then in the mobile market because of satration of above mentioned shovelware titles.
