John begins to care about his appearance after he spends the weekend at the Pigman's house with Lorraine. Chapter 12, in the book The Pigman, does not give much detail as to why John has his hair combed or is wearing a clean shirt. The chapter makes mention of John being charged up about Loraine's and his weekend at the Pigman's house.
Chapter 11 reveals more details to support John's sudden interest in his appearance. John and Lorraine had the Pigman's house to themselves. The two had spent time together making a spaghetti dinner. Lorraine had found some old dresses and dressed up in one of them. John had started to kiss Lorraine in the bedroom, but she had them return downstairs. John was still eager for the kiss. He had started to like her in a different way, and he thinks that she is lovely.