How much is "very early"? What are the standards for the Phantom, and who are setting those? What is the track record?
Of course if you get it off the box cracked already; or get a returned "new" shell cracked; or get cracks after 3 or 4 (or even 10) "normal" flights, that can be considered early and you probably got a defective unit (my guess is a bad plastic mix or issue during molding/curing phases).
Can happen with shells, batteries, processors, sensors, controllers, etc. Take it back to DJI, the seller, whoever, by all means. Or get it fixed yourself and learn a new skill.
Other than that, it's all well within expected failure parameters of most RC/hobby devices. Devices that cost a lot more and do a lot less than the P3, just to set the record straight. Search around.
Is it "normal" or "acceptable"? Certainly not by pilots unable to fly their ACs due to any issue. Is it "common"? Absolutely. This is an advanced and sophisticated RC flying aircraft not a mobile phone (those have issues too) or a DVD sitting on a shelf (ditto) doing only only one thing.
I have no idea about the percentage of cracking shells, but I'd be willing to bet it's lower than crashing drones anyway. Which is also well within the parameters of other RC/hobby models btw.