Just like humans, cockatiels can also suffer from baldness. It's nothing serious life threatening, that is just how some cockatiels are. Baldness on cockatiels is shown in the crown of the head and more specifically on Lutino cockatiels.
If your cockatiel has an aggressive playmate, the lack of feathers on your tiels head might be attributed to the other bird plucking the feathers. Do also check to see if your bird might have any skin parasites. Anytime you have an aggressive cockatiel plucking feathers to a companion, is best to separate them.
As always the information offered here is to provide guidance and is not intended to be a substitute for the good advice provided by your own avian vet. When in doubt always consult your own veterinarian.