Hit on the head. Would it do any damage to the transplanted area if it happened 30+ days after the surgery?

It is unlikely that 30 days after your hair transplant surgery being hit on the head would really affect the grafts. It would affect them just as much as the natural hair that was there to begin with.

Trauma to the head may be trauma to the transplanted hair or even native hair.  If you believe the trauma to the head was enough to cause damage to normal non-transplanted hair, it would also damage the transplanted hair.  Otherwise, it should be OK.

If the injury is severe enough, scalp injury could cause scalp inflammation and scarring regardless of whether a transplant was done or not. At day 30, the transplanted follicles (beneath the scalp at that point) have the same susceptibility to injury as normal scalp hairs But if injury is severe, scarring can occur. 

At one month, hitting the head will not cause any problems provided that no gash or cut in the scalp occurred. 

Usually most of the grafts are settled in mad the follicles are inside the skin after three weeks. The chances of damage to hair grafts after three weeks is very low.