Hi I have a small theory about why Frask became stupid:T I think this is caused by the red smoke in the mountain, something that supports the theory is when Pandora in the 9th Chapter use a anti gas mask to protect herself of the smoke, also when sans gives the vaccine to Frask and recover they memory and intelligence which can be too another effect of the red smoke, it's another proof, and the final proof is that the serie starts right at the mountain summit and as seen in the 9th Chapter the smoke is in all the mountain and Frask must have exhaled so much smoke that he became stupid and forgetful remember, that humans are intelligent in the AU

Snick