Here are some options depending on the context:
More neutral:
* Naive
* Unsophisticated
* Uninformed
* Clueless
* Slow (in terms of understanding)
* Misguided
More humorous:
* Silly
* Goofy
* Airhead
* Ditz (typically used for women)
More polite but still implying a lack of intelligence:
* Not the sharpest tool in the shed
* Slow on the uptake
* Not the brightest bulb in the box
Important note: Even these alternatives can be hurtful, so it's best to avoid using any language that might be considered insulting. Consider if there's a way to express your point without resorting to name-calling.