Are chiari 1 malformations ever acquired from a head injury or are they something people are born with? Is a 8mm chiari 1 malformation concerning?

Typically congenital. Many of these are congenital. It is essentially an area of your brain (cerebellum) that is partially herniated thru the large hole in your skull (foramen magnum). Some can be acquired (type 1) thru infection or trauma or things that result in elevated cranial pressure. Many congential type 1's are not symptomatic. If having c/o of headaches, weakness, or other symptoms i would see a neurosurg.
Congenital. This is a congenital condition. However, trauma can trigger the symptoms. Extent of cerebellar herniation is less important than the degree of discomfort and disability someone has from it.