dim bulb

A dim bulb is a person who lacks intelligence or quick thinking.

The expression draws on the metaphor of an incandescent light bulb that glows weakly due to low wattage or failing filament—such a bulb provides poor illumination, just as a dim bulb of a person provides poor thinking.

He's a nice guy but kind of a dim bulb.

The phrase is mildly insulting but usually delivered with affection or gentle exasperation rather than real hostility.