Caffeine is a drug just as nicotine is a drug. When you go off of a drug you are going to go into withdrawals. Although this physical reaction may be unpleasant it is not the same as a mental disorder. Mental disorders don't go away once a chemical leaves your body.
There is no reason to label caffeine withdrawal as a mental disorder. Caffeine withdrawal, at it's worst, tends to cause headaches, for only a short amount of time. There is no reason to waste time or effort, categorizing this problem. Mental disorders tend to revolve around conditions that are more serious than caffeine withdrawal.