Tabitha's kind of like the velociraptors from Jurassic Park—long before we actually see her, we hear a lot about her. And we're scared.
Early on, Jake calls her "that old bat from the New York Times" which tells us right away that she probably isn't such a nice, cordial person (no offense to bats). Later, in the bar, Mike uses a very colorful expression to describe the way she looks, which we're going to paraphrase here as "looking like she's been sucking on lemons."
And when we finally meet her, we can't exactly disagree with these less-than-flattering assessments. She comes off just as self-obsessed as Riggan is, preoccupied with the pretentious reviews in which she flaunts her own knowledge of theater…and not at all concerned about the careers she ruins in doing so.
Riggan's critique of Tabitha—that her criticism is nothing more than "labeling" things— is a reaction to her stating very matter-of-factly that she'll kill his play. Riggan's confused at first; she hasn't even seen the play, how does she know she won't absolutely love it?
But it's not the play Tabitha has a problem with, it's Riggan himself and everything he stands for that she despises. She tells him that he "took up space in a theater which would have otherwise been used to put on something worthwhile"…ouch.
And she doesn't even stop there. She can't take the artless narcissism of Hollywood, and refuses to even give it a chance in her domain. She describes the big movie business as people:
TABITHA: […] giving each other awards for cartoons and pornography. Measuring your worth in weekends.
Okay, so maybe opening weekend gross earnings isn't the best way to assess the worth of a movie, and maybe Hollywood tends to play to our primal desires in order to sell movies to the masses. We as viewers don't hate Tabitha because she's wrong, we hate her because she won't even give Riggan a chance to break the mold and do something different and meaningful.
In the end, Tabitha comes around…sort of.
She gives Riggan's play a stunning review that makes the front page of The Times and is sure to send it off on a enduring career of reproductions and adaptations. But for all that, the review is still full of words that are big in size and small in substance.
For all of her witty connections between the performance and its tragic end, she may have missed the point.