Does adding cocoa or chocolate flavoured stuff to milk reduces the quality of milk?

No: If it helps you to be better about drinking milk to add some chocolate—more power to you. A good compromise is to mix milk with something like powdered chocolate instant breakfast which has slightly less sugar but the cocoa itself does not keep you from absorbing the good qualities milk possesses. Cocoa adds some empty calories and a bit of caffeine but has no chemical effect on milk. Enjoy!