Well that is because she lacks content, when we get more and learn more about her then we can decide if her backstory is complex/interesting enough. I, for one, think her backstory could be moderately intriguing since there's the whole thing going on with her connection to Bella, and I feel like it has something to do with a tragedy of Jill's past, otherwise Bella wouldn't be so overprotective.
And I think all the Jill hype comes from what we've seen so far, she seems to have the potential for best girl but she has too little content. But I guess a lot of people here love a sweet and kind princess.