Yes it's okay to use Desitin diaper rash cream or any other diaper rash cream on a newborn and it will help prevent diaper rash by creating a barrier against the pee and poop from the diaper.
Although disposable diapers today really are very absorbent and hold the wetness away from the babies skin it's still a good idea to use diaper rash cream to protect against diaper rash.
Newborn babies skin is pretty sensitive and delicate and can get a rash from the urine or poop so using diaper rash cream or Desitin can help protect against that.
You don't have to use the diaper rash cream if you don't want to everyday or every diaper change but doing so certainly won't hurt the newborn.
Make sure you change the newborns diaper often as in every hour or 2 hours or when it's wet enough to need changed and if the newborn has a poop diaper then you should change that as soon as you notice.