When Misty states that Opaline has been "all alone" for so many moons, you can see a subtle change in expression. Opaline goes from being outraged, to a more hesitant expression.

This could show that part of her does in fact know that what Misty says is true. She regrets being alone for so long. Misty's words cut deep, because they're true, and for a brief moment, she seems to acknowledge that.

But once Misty expresses sympathy for her, she goes back to her usual self. She probably can't stand the idea of being pitied, and this makes her put her guard back up. She probably doesn't want to be seen as weak, even if what's being said is true. Her pride prevents her from actually realizing she could change and feel true friendship.

Obviously we see that she then continues to go down her evil path, eventually sealing her fate. Maybe if somepony had shown Opaline kindness earlier on, she could have had a chance at genuine friendship.

Now that she's trapped in the Together Tree, she's truly alone. Only her own thoughts can keep her company. Spending so much time trapped in such a way could very well be the thing that finally causes her to fully reflect on herself, if she just lets down her walls.

The fact that the animators decided to add this subtle change in expression shows that there's more to Opaline's characters behind her narcissistic tendencies and view of superiority.

Had MYM continued, I would have loved to eventually see more of her backstory (the full, unbiased one) and learn about why she felt the need to go down the path she did. Maybe she caved under the pressure put on her in Skyros, and felt that she couldn't live up to the expectations given to her. Therefore, she then decided to blame others (Celestia and Luna) for her problems.

Of course, with the new information about how Opaline was going to return in a future MYM season, and possibly even had a redemption arc, definitely makes this small scene seem foreshadowing of some kind, but alas, we'll never know what would have happened