Bad hair days happen. Along with an uneven shave or misapplied makeup, subtle missteps in appearance can be evidence of your loved one's struggle to cope with brain changes.
Should you mention it when something seems askew?
Only if necessary and then only in the nicest, most casual and respectful way -- as you'd mention, say, spinach in a dining companion's teeth.
Why isn't correcting helpful?
Your loved one is probably well aware of not feeling like his or her old self lately, and needlessly linking every personal-care lapse to dementia is apt to only make her or him feel self-conscious or upset.
What works better?
If you see your loved one struggling with an aspect of personal care, see if there's a more global way to solve the problem. Examples: a shorter, no-fuss haircut, a new paler lipstick or gloss that makes misapplication less obvious. Offer to help out in a casual way: "Here, maybe I can try that."