There is no such thing as "just a unicorn" Or "just a pegasus" or "just an earth pony" or "just an alicorn". Twilight has a wonderful destiny~! And it's no more or less important than every other pony's destiny
A character cannot have a wonderful destiny. Destiny denies free will, which in turn denies good and evil. In a universe with destiny, all outcomes are set in stone, and choice is impossible. A destiny isn't good or bad, it just is.
Well, even destinies they are determined it doesn't mean that they can't be wonderful And what a pony has for a cutiemark is only a small part of what they do for the world. Everytime someone has a conversation or performs an action that act could be carried from person to person and who knows what a few words might have on a pony's life. There's just no accounting for a person's worth in the world because there's no way of tracking and tracing the journey of every contribution they make no matter how small they may seem
I believe the universe does have a randomness and choice, just that maybe the creatures who live within it are drawn to doing certain things but it's them who choose how they do the things they do and what words they choose to say to each other The magic that makes a cutiemark is mysterious to us and maybe to them too. And a cutiemark or a creature's destiny is only a small part of who they are.
If I, as an outside observer, see a character living in a universe with destiny, I often pity them for not being able to make any choices or have responsibility.
If I, as a viewer, see a piece of fiction which uses destiny, I think the writer is a lazy hack who couldn't be bothered thinking through events and coming up with a plausible set of outcomes. That, or there was executive meddling involved, which is arguably worse.