He was in special classes.
Weed makes one forgetful and a bit slow in the mind, but even a totally baked stoner shows evidence of how much intelligence they really have.
Kevin is just stupid and high. He is basically a real life Ricky from the trailer park boys, without the ability to fight, grow weed, confidence trick cops, drive in a car lacking a door for actual decades without dying, work as an assistant, and temporary relief trailer park supervisor, or even demonstrate the creativity to mint his own currency out of hash.
A comedy character is essentially a better version of Kevin. That must be awful.