Here are some alternatives depending on the context:
* Informal:
* "Tonto" (silly, naive)
* "Bobo" (foolish, clumsy)
* "Nego" (dumb, clueless)
* Formal:
* "Ignorante" (ignorant)
* "Insensato" (unwise)
* "Despistado" (distracted, absentminded)
Always consider the context and your relationship with the person you're talking to when choosing words.