There are two possibilities I think here because there is a fundamental difference in the systems. Switch has two difference performance modes going by this rumor which most expected.
Scenario 1: Most, if not all developers, will target the lower of the two. Outside of Nintendo and a few major third party efforts I don't see many devs investing a whole lot of resources in implementing major improvements to their games when the system is docked. That means they will probably used those performance gains from docked mode to make their game run at 1080p, as all they'll really be doing is increasing the resolution and nothing else for their game. They likely won't be implementing more complex lighting, better shadows, textures, draw distance, physics, AA, etc. But like I said, I would imagine many, if not most Nintendo games will be designed to take advantage of the increase performance outside of just 1080p, as will some big budget third party titles, but probably not many of those unless the system is real success.
Scenario 2: Most devs target docked modes specs and then try to cut shit when undocked in order to gain acceptable performance. In this case devs will act like they do on other consoles where they look to push the boundaries of the system in order to make their games look prettier at the cost of performance. Which in this case will likely result in poor running games in both docked and undocked modes. At least for third parties, Nintendo will likely have rock solid to at least acceptable performance for most games.
I honestly think scenario 1 is more likely, and I also hope that is the case.