Try to be realistic...
If the DF-21's warhead descends from a suborbital altitude, it will be enveloped in a plasma cloud...
Space Shuttle thermal protection system - Wikipedia, the free encyclopedia
Plasma is not friendly to EM waves...
Piercing the Plasma: Ideas to Beat the Communications Blackout of Reentry: Scientific American
That means during the reentry, the DF-21's warhead will be blind if it has any type of sensor such as radar and/or infrared.
If this warhead is supposed to have some maneuverability during its descent, it
MUST slow down in order for the sensor suite to work, let alone pick up anything, once the plasma cloud dissipated. The greater the descent speed, the less time available to make any course correction, therefore, most likely the warhead will have only one possible course correction.
From the time the warhead is at apogee to when it begins its descent, the fleet will have it on radar. In less then five seconds, the fleet can deploy enough chaff to create a radar view of several thousands square km. Same for IR with flares.
Sippican, Inc. - Offboard Countermeasures and Training Systems
Rafael Anti-Ship Missile Defense System - Decoys
Now do not nitpick the fact that these sources present themselves as 'cruise missile' defense. Chaff/flare decoys present themselves whenever there are transmitting/receiving sources from any direction, even if the sensor suite is from above.
By the time the warhead is free of the plasma cloud, its sensor suite will see nothing but a large electronic blanket below it. What is there for the warhead to make a course correction towards a target? Keep in mind that under this electronic blanket, the ship is also moving. Aircraft carriers conducting flight operations do not travel in a straight line but into the wind, that mean this prized target will be making unpredictable maneuvers.
Still think it is easy to hit a moving target with a ballistic weapon?