This might be a regional thing - Here in New Zealand, exposed nail ends would be a sign of lazy workmanship using overly-long nails, or an overpowered nailgun here causing over-penetration and likely leaks.
Here in New Zealand, roofing fasteners are required to go into a purlin or similar, and missing the target happens, but leaving it would be poor workmanship.
I'd say not normal at all but your locale/region may differ. Possible solutions:
hammer the nail ends over so they are somewhat sunk into the wood, but this risks upsetting the seal on the top and causing a leak in the future.
use a hacksaw to trim every nail that you can get to. For thin wire-nails you might only need to use pliers.
use a small grinder with a cutoff wheel to do the same (but sparks in your roof space are an absolute definite fire hazard so consider your risks)
Insulate and line and ventilate the attic space so its more useful - the insulation and lining will "thicken" the ceiling and cover the roofing nails. Kind-of overkill solution.