Annette,
We have two Cajun French terms you might use:
Fais-drole (Fay drawl) is someone who acts strange or who does funny;
Pas-tout-la (pah too lah) is someone who isn't "all there."
Now, sometimes we call clumsy or 'slow to catch up' people "pauvre bête" (pawv bett) which literally translates into "poor beast." We generally regard farm work animals as being generally not the smartest creatures on earth, and we transfer that meaning to people that are dumb as a post.