Bright or not...he had three women attentive to his needs and desires.
As I've posted before, if the murderers didn't make any mistakes we wouldn't have the clues that lead to the "Columbo" resolutions of the episodes.
I think if you have to categorize the murderers, some were obviously really pre-planned; some were more spontaneous.
The ones who thought of themselves as really intelligent and above every one else intellectually were the most stupid on a relative scale. Of course with hindsight, we see how obvious the revealing clue becomes.
We also are probably lulled into thinking that certain little things that first bother Columbo would be obvious to us and stupid to have been overlooked by the perpetrator.