Does anyone else's loved one with dementia complain of constant headache?

If she has been taking high doses for a long time she might be getting rebound headaches... read up about them on the internet.

Be careful when giving regular and/or frequent doses of acetaminophen (Tylenol) as it can seriously damage the liver or kidneys - especially if acohol is consumed regularly- and not necessarily in large amounts. As it is, decreased liver and kidney functions is typical among the elderly. One common but sometimes challenging issue to diagnose among people with dementia that causes headaches is a change in vision. Another is a build up of ear wax (eww) as it can cause pressure in the sinuses.

My mom gets headaches and has had many ct all are normal ..her neurologist said just give her tylenol

Any complaint even from a person with dementia needs to be taken seriously and investigated. You have no way of knowing whether she really does have a headache or not. Talk to the Dr and request investigations a CT scan or MRI are both non invasive and will rule out serious things like a brain tumor. If something is found and she is not a candidate for intervention her pain needs to be properly controlled and numerous Tylenol is not the way to go. She may actually need a narcotic.