sharp as a sphere

Someone who is not too sharp, smart, clever, or anywhere near brainiac status.

Gill: Oh, I see you got your laces into your new shoes, Kev. Good Job!!
Kev: It was no problem, Gill. I remembered from last time.
Gill: You sure are as sharp as a sphere!
Kev: You bet I am! Now help me tie my new shoes.