My wife asked me to put ketchup on the shopping list that I was making...

 My wife asked me to put ketchup on the shopping list that I was making and now, I can’t read anything.

This is a joke about a misunderstanding, and also about stupidity and misfortune. The misunderstanding is that the person telling the joke apparently misunderstood their wife, who was obviously asking for them to add the word ketchup to the list (by writing it at the bottom of the list), not to smear ketchup all over the list.

Obviously the person that did this would necessarily be stupid, and the misfortune comes in to play because the person's stupidity caused the person to ruin the list that they were making.