Sure, everyone in the novel is filtered through Scout's perception. She's the narrator, after all. But we get the sense that Calpurnia in particular is colored by Scout's perspective—and her perspective sounds a little like Cinderella thinking about her wicked stepmother:
She was all angles and bones; she was nearsighted; she squinted; her hand was wide as a bed slat and twice as hard. She was always ordering me out of the kitchen, asking me why I couldn't behave as well as Jem when she knew he was older, and calling me home when I wasn't ready to come. Our battles were epic and one-sided. Calpurnia always won, mainly because Atticus always took her side. She had been with us ever since Jem was born, and I had felt her tyrannical presence as long as I could remember. (1.12)
Scout at first sees Calpurnia less as a human being than as a force of nature that she runs up against all too often, someone who wins their battles not because she has right on her side, but because she has the might. That's why she totally misinterprets those moments when Calpurnia softens up:
Calpurnia bent down and kissed me. I ran along, wondering what had come over her. She had wanted to make up with me, that was it. She had always been too hard on me, she had at last seen the error of her fractious ways, she was sorry and too stubborn to say so. (3.75)
Caught up in the tunnel vision of her own perspective, Scout can't see that Calpurnia is hard on her because she cares about her—just as much as Atticus, in her own way.
When Calpurnia takes the Finch kids with her to First Purchase Church, the kids get a whole new look at their cook. For one, she talks different:
"Cal," I asked, "why do you talk nigger-talk to the—to your folks when you know it's not right?"
[…] "Suppose you and Scout talked colored-folks' talk at home it'd be out of place, wouldn't it? Now what if I talked white-folks' talk at church, and with my neighbors? They'd think I was puttin' on airs to beat Moses."
"But Cal, you know better," I said.
"It's not necessary to tell all you know. It's not ladylike—in the second place, folks don't like to have somebody around knowin' more than they do. It aggravates 'em. You're not gonna change any of them by talkin' right, they've got to want to learn themselves, and when they don't want to learn there's nothing you can do but keep your mouth shut or talk their language." (12.139-144)
In other words, what's right in one place may be wrong in another. But is that always true? Atticus is known for acting the same everywhere, and that's presented as a good thing. Why can Atticus always be the same, while Calpurnia has to adapt to the community she's in? (Hint: it probably has something to do with a little thing called white privilege. Not to mention male privilege.)
Seeing Calpurnia in relation to the African-American community makes Scout realize for the first time that Cal actually continues to exist when she's not at the Finch house:
That Calpurnia led a modest double life never dawned on me. The idea that she had a separate existence outside our household was a novel one, to say nothing of her having command of two languages. (12.138)
So now Scout is curious, and she peppers Calpurnia with basic questions like when her birthday is (she doesn't actually know, not even the year) and where she grew up (near Finch's Landing).
But while Cal shares the basic facts of her life, we don't learn how she feels about them. Does she miss her childhood home? Was she happy there? Did she leave family members behind? What does she do on her days off? Scout does learn to see Calpurnia as a real person over the course of the novel, but it's still an open question: how much does the novel give Calpurnia a real identity, and how much does it just push her into the role of wise black woman?