Applejack seems one-dimensional because all she does is work. Her life literally revolves around hard labor. It's what she does.
If I hadn't seen Apple Family Reunion, but seen its summary, I would have initially thought that it would have become Applejack's Lesson Zero. It subjects Applejack to what may be the two things she fears most: failure and disappointing her family. But instead of freaking out like Twilight does, Applejack internalizes it.
Her character growth since season one has been almost zero because Applejack is rarely the focus of an episode and when she is, she either doesn't learn anything (lawl) or she doesn't learn it because of her pride. The only reason she accepted Twilight's help in Applebuck Season is because she nearly worked herself to death and her lack of rest was starting to affect the other townspeople (townsponies?). Applejack did the same exact thing in The Last Roundup, choosing to completely abandon her friends and family, worrying them to the point of chasing her to Dodge Junction just because Applejack was too proud to tell them the real reason why she wasn't coming home on schedule. The two episodes have completely identical lessons, and she didn't follow them in either episode.
I don't know what Granny Smith was thinking, letting Applejack run the reunion all by herself. Seriously, all by herself? Granny Smith could have at least guided Applejack's planning, or acted in a supervisor role, even if it was just a minor one. Throwing her into the fire like that, it's no surprise Applejack's planning sucked.
But anyway. When the barn first came down (side note: it couldn't have been that sturdy in the first place if a swarm of bats brought it down - either that or those are some strong-ass bats), Applejack was clearly ashamed of what she'd done. But since she was put on the spot, she couldn't run and hide, so she came up with a plan, which basically boiled down to 'whistle while you work.' I don't know why Applejack thought this would bring the family closer together. If anything, you'd think they'd all harbor at least a little resentment for Applejack for making them rebuild the entire barn from nothing but wreckage, and in just a few hours (presumably)! Man those Apples can work fast. Towards the end of the song, one of the ponies looks at another one of the ponies while they say "yes we did", like "did we really just build a barn in one afternoon? is it gonna hold? also, how did I know the lyrics to this song?"
The lesson here wasn't even one that will make Applejack a better pony, making this episode--in the grand scheme of things--moot. (From a logical standpoint it wasn't even that great an episode, but it gave me the Applejack song I've been fiending for for like a year so all is forgiven.)
What we need to see is evidence that Applejack is actually learning the lessons she's meant to learn, not reverting to her old standbys in every episode. It's similar to what bacon said about Fluttershy going back to her old mannerisms within an episode or two. We know that there is some loose continuity in the series but some characters like Twilight and the CMC are getting all sorts of character development while Applejack does not. Even Rarity's had a little growth.
Applejack does the same thing in every show except for the examples I outlined above: run and hide or, failing that, sing and hope to god no one notices you just ruined the reunion because I know I didn't come here to build your stupid barn, I came to have fun.
I mean, I love me some Applejack. If I wasn't so in love with Twilight Sparkle and Rarity, Applejack would definitely be at the top of the best pony list. Everything about her is sexy: she's strong, loyal, and dependable; she can cook like a boss and knows how to run a business. Unfortunately, all we usually get to see is the strong part. Okay, we get it, she can work hard. Take her out of her comfort zone once in a while. Show us she's more than just some finely toned legs, blonde hair and a hat.