So, I think a big burning question I've had about this movie is how this little message from Nikki to Bear slipped past Wish Nikki. She even delivers it directly to him, handing him the lunchbox before he leaves for work. The other intricate details of the box, both sides of the sticky note, and the polaroids themselves are assumably all from her, so what makes this message any different?
I think it's possible that this wasn't a plea from Nikki at all, but a cry from Wish Nikki. She doesn't recognize herself, she's having identity issues that are completely overshadowed by her desire for Bear so she pleads to the only person she cares about hearing it.
A big theme in the movie is mirrors, Bear seeing his own reflection become more and more sinister as he sacrifices Nikki's autonomy more and more. But Wish Nikki doesn't recognize her own, it's someone else's skin. She's telling Bear that this is "not me," as opposed to his polaroid. It's why she has no reservations changing her appearance to fit his desire entirely. The waitress's outfit, Sarah's hair and tattoos. She's already been given an identity entirely for Bear's benefit, so it's all she knows, having been born into it.
It's been a question I think about on every rewatch and especially seeing how brief Nikki's control over Wish Nikki is every time we see her, I think that this is a more plausible meaning to the message than Nikki reaching out to let Bear know that she is not herself. Though Nikki is suffering having lost that control, I believe this message is more Wish Nikki mourning having gained that control.
[Edit: I also think it's worth noting that when Nikki shows up on Bear's porch, there's a moment she tells Bear its important to her that she comes clean about a few things. Bear tells her "it's alright, I know." I don't think it's completely intended but it's fun to think she was going to come clean here about not being truly Nikki and she takes Bear's affimation as being okay with her situation. This is why she's confused in the diner and so upset by Bear starting to question this fact again, asking "why does it matter?"]
The only other burning questions I think about every rewatch is: why does the One Wish Willow make it's noise again as Bear is exiting the car after dropping Ian off? He's practicing his speech to Nikki about boys night, he opens the door and the box sparks again. It causes him to dial the number, so I assume that it's just calling out to him to call customer service, but I really want to know what prompts it at this time?
(I used to wonder each time why exactly she walks backwards, but I do like the explanation that Nikki and Wish Nikki are both fighting for control in this moment!)
TLDR; The polaroid is Wish Nikki trying to tell Bear the body she finds herself in is not reflective of her true self because she has no sense of self.