The joke itself is about Sir Tristram, and how he attacks people apparently without much regard for his own safety or without parleying first. Obviously this isn't a serious claim, but it got me thinking: would a person who is considered disabled or physically impaired in any way today be able to become a knight in the past?
Much more can be said, especially about knights with physical impairments, but for a degree of information about vision see:
How did humans cope with poor eyesight in the Middle Ages? Did all hunters and knights thus have 20/20 vision? written by u/sunagainstgold