Draco's going bald because Rowling wanted to throw in that old cliche that your high-school tormentors are the ones who turn up bald and/or fat at the class reunion while you, of course, look fabulous. She's displayed a keen understanding of adolescent-level schaudenfreude throughout these books and I'd guarantee there isn't a kid out there who didn't derive catty delight from Draco's premature baldness.