I'm not really sure what your point is?
If Jackie was the main, it'd be a completely different show, as she's a very different kind of character. I don't think she'd make the same choices Star has made, because she's not Star.
You kinda seem like you don't understand why someone would like a non-main character, so you project like an i-max. I don't love Jackie because she appeared so little, I love her for her kindness, positive cheer, maturity and sportyness.
You kinda seem like you're placing way too much weight on how big a role someone played in the story or "affected the story", and imagine that people only like "meaningful" or important characters (thus, if someone likes a character, they must think that character is super important in order to be worthy of liking) but your'e dead wrong.
Jackie makes me feel happy, that's all that it is. There's just something about her earnest positivity.