he is constantly being represented as uneducated and dumb

Because of his low intellect.

Try reading the book. It's less than 200 pages for crying out loud.

he is constantly being represented as uneducated and dumb

George is always manipulating him and overpowering him with his intellect. Then when they’re on the ranch Curley threatens him then attacks him. When he goes into the bunkhouse where Crooks (the black man) lives, crooks also starts manipulating him and tormenting him by suggesting that George might leave him. The people in the novella often take advantage of him because he has a lower intellect