The main character in Hunted by James Alan Gardner is less than bright, yet still likeable. It affects the plot. There are comedic moments in the book, but it also deals with serious themes, and the character, Edward York, is not just there to be laughed at.

The private detective in Anthony Bidulka's mystery novels, starting with Amuse Bouche, seems to me to be a little dim, and somewhat flaky, but the book doesn't make a big deal of it.

Also, there are various main characters that start out young and stupid, then grow out of it. There are also various novels where the main characters do incredibly stupid things. I can think of a few examples. But I'm not sure that's quite the same thing.