The term has long history and painful history, not unlike that other word that starts with an 'N' and refers to or denotes
anti Blackness. "Nappy" hair has always been a signifier of "otherness", and has more often than not been used derogatorily, due to it's negative connotation. It's yet another thing that reinforces the fact that we are not white. Worse yet, it's still used as an insult amongst Black people, usually lobbed at Black women to point out perceived unattractiveness. Nappy hair is hair that is (traditionally) seen as unkempt, dirty, unmanageable and ugly, therefore it is considered to be undesirable for a lot of Black people.
Even with the emergence of the natural hair community and the complete change in how Black women (in particular) look at and think about their hair and beauty, that one term is still as divisive as ever. There are some who have reclaimed and embraced the word in an attempt to strip it of its negative meaning, and if you look at blogs that focus on natural hair, you'll find a lot of blog titles and pennames that include the word or some variation of if. Acceptance of the word has gone a long way towards helping young Black girls appreciate and accept what's on top of their heads. And that has been a good thing because self-love and acceptance is especially important for the next generation, we need to be working to avoid mistakes of the past.
Apparently (because you knew the bullshit was coming), it has become quite a common trend for white women (actually non-Black people in general) to use #NappyHair on Instagram and Twitter. This is yet another example of cultural (mis)appropriation and why there is so much complaint about terms commonly used in Black culture, now being used freely amongst non-Black people. It's not that only Black people should be using these words, but that when non-Black people use them they are used incorrectly and without thought or respect to the history of the word and its connotation. A white girl with straight hair describing her uncombed bedhead as "nappy", is kind of insulting. It further pushes the term right back into "undesirable" territory.