Ostensibly, Sue Ellen could be considered dumb for her second wedding to JR, but I see that as more her dysfunction and addictive behavior than lack of intelligence. Smart people can make dumb choices.
There were times Lucy was written as shrewd and intuitive, but these were few and far between, unfortunately. For example, she stayed by Bobby's bedside when Katherine came to visit him because she knew something wasn't right about Katherine's visit. She probably saved Bobby's life.
As far as stupid goes, I'd say Holly Harwood for going into business with JR without checking into him first and giving him a whopping 25% of the company. Like, wow.
Honorable mention for dumb goes to Miss Ellie. For all the hand wringing she did over not getting to see Gary, she never once in all the seasons thinks about getting on a plane and going to visit him. Like, wow part 2.