"Is a mood" (not "is in a mood", which is a more standard and older, more widespread expression) is modern American slang.
As an almost middle-aged British English speaker, this variety of slang isn't part of my native vocabulary and I can only try and work it out by the context.
The way I interpret "...is a mood" or "...is a whole mood" is that the particular person or action is engaging in an action or mode of behaviour that epitomises a particular mood and deserves to be classed as a whole mood of its own rather than subsumed within an existing mood-describing adjective such as "sad", "grumpy", "angry", etc.
TheMarySue recently had an article titled "Sebastian Stan Yelling About People Still Going Outside Is a Mood".
There was recently a similar question about what "...is a mood" means on Quora. One answer was "When something/someone does something so relatable that it is an entire mood". Another was that "If you say 'something is a mood' you attribute an atmosphere or an emotion to something physical".