More general:
* Foolishness: This emphasizes a lack of good judgment or sense.
* Ignorance: This focuses on a lack of knowledge or understanding.
* Inanity: This emphasizes the lack of meaning or purpose.
* Obtuseness: This emphasizes a lack of sharpness of mind or intellect.
More specific:
* Dullness: This emphasizes a lack of mental sharpness or intelligence.
* Frivolity: This emphasizes a lack of seriousness or substance.
* Recklessness: This emphasizes a lack of caution or thoughtfulness.
* Thoughtlessness: This emphasizes a lack of consideration or empathy.
The best choice will depend on the specific context and what aspect of "stupidity" you want to emphasize.