Will diaphragm palsy get better after thoracic surgery?

It certainly can: Just like everything in medicine the outcome of any particular event is highly dependent on the amount of damage or insult that was done in the first place as well as the current state of health of the individual we are talking about. In the case of diaphragmatic palsy there are certainly rehabilitation exercises that can be done and often times they are successful at least to some extent.

Possibly: It depends upon the type of surgery, how long the diaphragm has been paralyzed and ultimately, the cause of the paralysis. If the phrenic nerve was injured or disturbed in some way, but not severed, the diphragm will ultimately regain function. If the nerve was severed, the paralysis is permanent.

Not likely: Diaphragm paralysis can be caused by trauma, infection, stroke, prior thoracic or cardiac surgery. There is surgery to address the condition, using plication to reduce the height of the diaphragm or pacing to make the nerve work. There is nothing about thoracic surgery which will make the diaphragm work if it was not working before, if that was not the intent of the surgery.