Generally agreed, OP. Sony's excellent first party games show that it's not Nintendo's great first-party games that are the cause of shitty 3rd party games (at least unless you want to say that 3rd party PS2 games were as bad as Wii's, and that it was Sony's first party games that caused this phenomenon...pretty clearly not true.)
3rd parties put out crap on Wii because they feel they can get away with it, and they were caught unaware by Wii's rapid success as a platform. I actually think N should start rejecting crap games and telling the devs to go back and fix them before they can be released on the Wii. 3rd parties would jump if N told them to, in order to release their game on the Wii. Even after fixing a game to N's satisfaction, the game would still have been cheap to make compared to an HD console game.
N's considerations should be: cost of inspecting 3rd party games and potentially pissing off devs vs likely income boost of having Wii 3rd party games be of a higher overall quality. Maybe N would rather just beat the shit out of 3rd party games in sales, though, since they make more money per unit sold for their higher-quality 1st party games.
