Can craniotomy (incision ear to ear), cause hair thinning 5 years post surgery?

Craniotomy scars will leave an area of no hair growth (on the scar) but it does not cause general hair loss after 5 years.

Your hair loss could be due to the craniotomy but could also be due to something else as well. The only way to determine exactly what is going on is to view the scalp up close. Please see a physician who specializes in hair loss. 

There is probably no relationship between your hair loss and your surgery 5 years ago; however, everyone who has genetic hair loss can find it impact them almost at any age. As you did not tell much about yourself, please for your sake, see a good doctor and get a diagnosis. With that a treatment plan can be created for you.

As a retired neurosurgeon who later became a hair transplant surgeon, I cannot think of any reason your craniotomy of 5 years ago would be causing hair loss now. My best guess is that the two are unrelated. I would recommend seeing a dermatologist to get some answers.