It's a joke. A student's report card is a card that says how well the student did. It often happens in western countries that when a child has bad grades, their parents punish them by forbidding them to watch television. The father asks his son why his grades are so bad. He would probably like to hear some good excuse so he could believe his son is not lazy. Instead, he hears the response, "I was getting tired of watching TV anyway." This shows a total failure of the father's motivational methods. It turns out that the son learned only as long as he wanted to watch TV. When TV started tiring him, he stopped learning, because there was nothing to motivate him.