I believe that this is fairly normal; I have done it myself since I was very young and I think that I've turned out fine. Oftentimes that imaginary world helps to take one's mind off of a stressful day or an event that has been worrying them. As long as she understands that this imaginary world is not real, she should be completely fine. She has a great imagination, and should embrace that - it will get her through a lot of tough times in her life.
She sounds so much like myself.
I'm a sixteen year old girl, who has always had bizarre and sometimes heavenly images pop into my head. I can't even remember how long I've had these images, but it seems to be since i was around four or so. I've always used these images as a way to calm me when stressed, and it honestly takes the stress away.
Now, i do suffer from panic disorder and OCD,but i doubt that your daughter has either.
I just think your daughter has a very amazing imagination, and i definitely think you should be a proud mother for how imaginative your daughter's brain is.
Good luck to you both :)
Not a Psychiatrist so I can't diagnose anyone. I can only give you my thoughts as a Psychiatric RN.
Worrisome symptoms would include auditory (most common) and/or visual hallucinations; seeing people or things that are not present or hearing voices in her head. I don't mean imagining dialogue between characters in her fantasies, but physically hearing voices addressing her as if it was a person speaking in the room. The voices usually start out complimentary and progress to demands to harm herself or others.
In my opinion, she sounds like a highly immaginative young lady that may benefit from encouragement to further express her creativity. Maybe suggest writing, drawing, painting, etc.