Popping or cracking your back in moderation is not bad and can help relieve tension and back pain.
Just don't overdo the popping of your back or cracking of your back because it could lead to more back pain.
Popping your back too much can cause excessive wear and tear on your back and spine and can lead to more issues and can prematurely breakdown your spine and back.
So as long as you only pop your back in moderation and not do it too often you'll be okay.
If you pop your back incorrectly though you might injure your spine though and possibly be paralyzed as I heard that had happened to some guy years ago.
He popped his back nearly everyday and then one day he popped his back wrong and then had messed up his spine which left him paralyzed.
So be careful and learn how to pop your back correctly.