what does Edgar Allan Poe's story 'A dream within a dream' about?

It's a poem, lovely.

As for what it's really about, no one but Edgar knows, and with him being dead and all, we may never find out. Instead, as with all poetry, scholars speculate, given the date of the poem, the events of the world at the time, and the personal happenings of his life at the time, and tell us what it's about.

I say, poetry is what you want it to be.

It's about whatever it reminds you of at the time you read it. It can change from reading to reading.

I think if you think of something, and find enough examples to back it up, you'll be golden.

I don't know. I never read the story, but if I wanted to know, I would read the story. If you want to know, you should read the story.