Jake's a casualty of the war Riggan wages against…well, pretty much everything: his old self, critics, Hollywood, his own dressing room.
As Riggan's lawyer and manager, Jake's tied up in the success of What We Talk About When We Talk About Love whether he wants to be or not. But that doesn't mean Jake has no stake in the success of the play. When Riggan's having difficulty keeping Mike under control (and keeping the play from going under before it even opens), Jake screams:
JAKE: This is about being respected and validated. That's what you told me; that's how you got me into this s***!
Because we get such an intense look at the mental and emotional stress burdening Riggan, we can forget that Jake's under similar pressure. The poor guy has to deal with the financial side and with the stark reality of the real world as represented by Ralph's lawsuit. (You don't just get beaned by a spotlight and not sue. Not on Broadway.)
And in order to do this, Jake will do whatever it takes, even if that means lying to his best friend about whether or not Scorsese is going to be at the premiere (fun fact: you can actually catch a glimpse of the maestro himself when Riggan comes into the theater in his underwear) or yelling at Laura and Lesley when they catch him in that particular white lie.
Because Jake's not lying when he says,
JAKE: I'm the one keeping this afloat!
Jake may not have the calmest of demeanors, but he's responsible for holding the play together by the skin of its teeth. Happily, in the end, Jake finally gets what he's been after. When Sylvia reads Tabitha's review she asks him:
SYLVIA: Are you happy?
JAKE: Happy? I'm f***ing euphoric…I've been reborn, brother, and I can see the future. This play is gonna last forever.
It's not that Jake's unable to see past Tabitha's meaningless labels, it's just that he's not after critical reception. Jake seeks financial success, and the long lifespan and commoditization of the play is the kind of respect and validation he craves most.