The problem
So imagine you are being entertained by a TV show set in medieval times, or a world with a medieval level technology (for instance, Netflix’s The Witcher) and suddenly you notice that while the characters hairstyles might look appropriate given the apparent level of technology, their hair is clean and immaculately conditioned, even if characters spend their daily lives on the road, and not horribly greasy and otherwise messy. This stretches the suspension of disbelief to (read: beyond) the breaking point.
To make matters worse, if one look at the clothes, one would see that while the style of the clothes may be appropriate for the depicted level of technology, if you notice the above mentioned problem, the clothes the characters will likely look brand new, no holes, no dirt and not wear. Even for characters that spend their daily lives homeless in a wagon. Again, to that this stretches the suspension of disbelief, is a rather severe understatement, and an obvious sign of low production values.
Not all shows make this mistake
An example of a show that makes this mistake is, as mentioend, The Witcher on Netflix. An example of a show that does not make this mistake, is Game of Thrones, which reflects its high production values, and is important for the viewer to remain enthralled.