Spider cracks are non-structural and caused by flexing of the shell, not impact. The outer gelcoat is more brittle than the underlying fiberglass, so when the shell flexes, the fiberglass bends but the gelcoat layer cracks.
Truck liner won't prevent them from happening. It's useful for the lower front, which is typically affected by impact damage, but it would do no more good than a coat of paint
elsewhere and be much harder to clean, wax, etc... Kind of ugly, too, in my opinion.
If the appearance is not bothering you too much, just keep your trailer clean and waxed and let it be- it's not really hurting anything. If it does bother you, paint
is the best solution from a cosmetic perspective. Lots of good threads on that, including the best kinds of paint that will flex with the shell.