Not Too BrightOne worth keeping
“About a half bubble off plumb.”
Not quite level-headed, like a crooked carpenter's level.
In the same vein
All Not Too Bright →Dense. All weight, no sharpness.
Looks alert. Isn't.
Not enough sense in there to do the smallest job.
Can't manage even the simplest task.
Not sharp at all.
He's lost whatever sense he had.