Birdman doesn't go for the cheap shots when it comes to Lesley. Sure, she's an actress…and this film doesn't have a whole lot of nice things to say about people who make their living before an audience. But she's also the voice of reason; a sensible woman with her head on straight.
Just check out how she schools Mike on his priorities:
LESLEY: Maybe up here you're Mr. Truth, Mike, but in the real world, where it counts, you're a fraud. How's that for truth, you dick!
She also, um, doesn't exactly hold back. She introduces Mike by saying that the two of them "share a vagina." Dang.
It's not like Lesley's immune to self-doubt or bouts of self-loathing. She's a little depressed that, now she's "finally made it" on Broadway, she feels like nothing's changed. There was no magic feeling of accomplishment like she expected. But what she's coming to terms with is a pretty normal feeling—kind of like the feeling of deflation you feel after opening all the presents on your birthday.
And we know that Lesley doesn't let her ego run away with her. Just check out this exchange:
LESLEY: Why don't I have any self-respect?
LAURA: You're an actress, honey.
If Laura had said that to Riggan or Mike, she would have ended up with a black eye. Instead, Lesley and Laura share a smooch.
In the end, we never get to see Lesley's resolution. Does she come to terms with her aspirations and her success? We don't know that, but we do know that kicking Mike out of her life will probably make her feel a lot better.