Some biographical information is here, and details along with informed speculation regarding his eye condition is here.
Thurber had one eye injured by an arrow in an accident when he was a boy. That eye had to be removed. His other eye developed problems and his vision began to deteriorate as he entered his forties. Eventually he was declared legally blind.
I don't have dates for the cartoons displayed below. By their appearance, I assume that they were mostly drawn before his eye condition had progressed very far.
Bottom line regarding Thurber: He should not be judged using professional criteria.