The emptiness in the SweetieBot's eyes is haunting because, while it could just be the empty stare of a machine, there's something more behind it. If the top part was all there was to the picture, then it would easily be just a picture of Sweetie Belle under Luna's wing. Putting a SweetieBot underneath with the dim light of stand-by in her eyes creates a sad contrast of the two, letting the viewer see that the puppet dreams of being real in a literal sense. The impact is in realizing that the machine has the capacity to dream at all.
>the puppet dreams of being real Sweetie Belle isn't fictional (well, in-universe anyway), so she is, by definition, real. Yes, I know what you were getting at, but still.
Things I'm curious about here/want your speculations on: 1) Can a robot pony get a cutie mark, as Sweetie here clearly hopes (check the far wall there)? 2) How/why did Scoots (they apparently met each other before meeting Apple Bloom) become friends with the creepy robot girl? 3) Think robots are another one of those things They conveniently "forgot" to update Luna on?
>Sweetie Belle [...](is) real. Okay, poor word choice; she wishes to be an organic lifeform of the sort that she resembles : P
Now you're asking for stuff that Griffon and Jenny would be better suited to answer, but you did ask for speculation too : )
1) I doubt it, but maybe she's hoping that, if she does something that would earn a cutie mark, then she might become a true pony ("true" in the traditional, puppet-related, "I wanna be a real boy!" sense). Or, she might know she would never gain one and is merely going through the motions to both help her friends gain theirs and to help herself have some way of feeling like a normal pony despite being a robot. I'm assuming that she knows she's a robot in this case. If she doesn't know, then both this comment and the previous comment I wrote are invalid and only the first applies because it's my opinion of how Sweetie Bot's eyes look : )
2) This is going into AU of the FiW AU, lol. Maybe Scoots saw Sweetie sitting off to the side watching their classmates and decided to try cheering her up, or Scoots wanted to know what it was like to be a robot or something. Kids are curious about things that are different and it's only a matter of getting past their fear of the unknown to investigate; some kids get past that fear easier than most of their peers.
3) Nah : ) If Luna is visiting Sweetie Bot's dream, she probably can feel the difference between a biological pony dreaming and a robot pony "dreaming". I imagine that Sweetie was dreaming of being teased or something similar, and Luna came by to investigate. When Luna realizes that the construct has the ability to access the dreamspace, it changes her perception of the idea of constructs being soulless and she offers Sweetie the opportunity to experience a dream where she can be a real filly instead of a robot.
Who knows? Maybe Luna found beings in her time that had Sweetie Bot's dilemma (golems and the like, constructs with magical origins instead of technological ones, unless robots and other "modern" tech run off magic...), so Sweetie wouldn't be much of a shock to her.
This is TL;DR already, but now I'm looking closer at the picture and I noticed this: Luna's tail and mane has the digital pattern instead of her usual stars. Is that really Luna, or has Sweetie coded her own for herself to interact with when she heard stories about Luna visiting the dreams of biological ponies?
Well what I got out of it was that since Sweetie Belle (or rather, Sweetie Bot) doesn't have a very good relationship with Rarity, who often times ignores or neglects her, she fantasizes about being Luna's sister and having a strong sisterly connection with her in her robot dreams.