As can be seen in the following link it is forbidden to practice with foam roller in the lower back. But is the situation is different with tennis ball?
The reason it's "forbidden" to foam roll your lower back is because you don't have a rib cage there to protect your internal organs.
You can, and often should foam roll your lower back, but it should be done with something larger than a foam roller to spread out the pressure over a larger area.
So no, do NOT use a tennis ball to roll the lumbar region of your back.
If you feel like you have a reason to do so, go get a proper massage. I would highly advise that everyone does this anyway.