He's basically so brain-damaged he can't even read any more. This might explain some of his obvious confusion in his Jackoff the Go videos, where he can't figure out what he's ordering and when it comes, he can't remember what's on his plate and has to ask Hammy.
It could be his vision, but probably it's just that the brain damage from the strokes has rendered him unable to read.
Or it could be both. About nothing about this semi-sentient blob of flesh actually works at this point.
That's also why he gets so angy all the time, it upsets him when he's literally incapable of even understanding what's going on around him because of the brain damage.