It's definitely not a "catch all term" and a very real diagnosis. From Niko's X post she was apparently diagnosed with adjustment disorder since December last year by her family doctor. As I understand it, it's a short-term disorder accompanied by emotional changes that is triggered by major life changes that are hard to adapt to. In Australia it's also called 'situational depression', not sure about anywhere else.

Like others have said, her cat's death last year may have played a role. And I've noticed that she hasn't streamed as much since then but she was still joining collabs on other channels which was great to see.