I spent some time thinking since the episode aired. Now we know she’s goth, she sounds kinda like Raven from Teen Titans and enjoys darkness. So it’s easy to just assume a Goth Girl wants the dress of the Moon Princess. Who wouldn’t?
 
But what does her cutie mark mean? My theory is her special talent is helping someone in find just who they need in their life to make them whole. Why? The cutie mark shows 4 crescents sort of pointing towards a purple heart. Seeking to help a wounded or tired heart. True love? Or maybe just find a pet that’ll help them get through what they need? Could even be she runs an Ask Moonlight newsletter in the Canterlot newspaper!
 
Why would this make her Goth? Well she sees people who are happy and perky all the time. Not everyone can be like pinkie pie and show off that boundless joy of life. Sometimes if all your seeing is smiles, joy and bright lives you just might start to enjoy the calm moonlit nights alone with a cup of tea. That’s what makes her cool. She could be someone so wonderful yet all we know is she feels like a princess wearing something styled after Luna.
 
Of course this is all headcannon and theory so feel free to disagree, agree, or just wish me to get splashed by Discord’s Exploding Chocolate Milk.