Well, they say she's a background pony because some of the episodes supposed to be centered around her don't really have much of her. Applebuck Season was an exception, as was Look Before You Sleep, but The Last Roundup and The Super Speedy Cider Squeezy 6000 are what I mean. The Last Roundup is about her, but we don't focus on much except her not giving a reason for coming home after the rodeo. The SSCS6K was more about the Apple family and the Flim Flam Brothers than Applejack. Again, I may be completely wrong, but that's my perception.
I also like Applejack. She's very sweet and likeable.