If you want an in-universe answer for MLP, I'd say that it's Celestia training Twilight to be her replacement. It works as a pretty good catch-all answer IMO.
Anyway, let's have a look at Celestia's role in the various big stories.
Friendship is Magic - Not present. Actually, where the hell was she?
Celestia was inside the Sun. After Nightmare Moon's defeat, Celestia's voice appears, then the Sun comes up (remember, only Celestia can do that), and Celestia appears to come
directly out of the Sun.
It's been theorized that Celestia was
imprisoned inside the Sun, just like Nightmare Moon was very clearly imprisoned inside the Moon, but it makes more sense for Celestia to have been
hiding in the Sun. First up, there's no obvious mechanic for Celestia to have been captured/defeated and imprisoned. Nightmare Moon breaks out of jail, and literally two seconds later Celestia's confirmed to be "missing". Everyone immediately blames Nightmare Moon (except fourth-wall breaking Pinkie, who says she's hiding), but NM doesn't claim credit for Celestia's defeat, she lets fools believe whatever they want to believe. And Celestia was playing Twilight like a harp. She admitted that she saw the threat of NM coming a mile away, but knew that Twilight was the only one who could solve the problem, so she manipulated Twilight. NM could theoretically be smacked down by a greater power, but one of her issues was envy of Celestia, so a Celestia beat-down would only make the problem worse.
Celestia hid in the Sun, her own domain, knowing that Nightmare Moon wouldn't/couldn't raise the Sun, and would therefore never find Celestia.
Return of Harmony - Not present. She can't wield the Elements anymore, so I guess she just chills in Canterlot.
Yep. She knows she doesn't have the power to beat Discord, but she acts like a leader, giving Twilight all the info she needs, and then handing out the big guns. Until... oops, Discord swiped the big guns. Celestia is revealed to be absolutely powerless against the overwhelming power of Discord, so all she can do is sit back and hope that the Mane Six can win somehow.
And then, as DrForester mentioned, Celestia went and collected all of Twilight's "Friendship Reports" for the past year (remember, RoH was created as a season 1 episode), making use of an ever-present part of the show to drop a friendship bomb on Twilight (at Spike's expense) and remind Twilight of who she was, in order to help Twilight keep her head in a fight against an enemy who can literally take your mind away from you. Twilight and the Mane Six were the ones who did all the fighting, but when all was lost, Celestia was the one who subtly tipped the balance and put the Mane Six on the winning track. It was all downhill from there.
A Canterlot Wedding - Put out of commission by Chrysalis.
This one's an odd case, because it was written by Meghan, but overseen by Lauren, who then promptly quit.
Celestia got smacked down for the first time, but at least they had the sense to play it as something shocking and surprising.
Cadence got upgraded to Princess by Hasbro, for no good story reason (it works just fine with Cadence as a Unicorn), and against Lauren's wishes.
And this episode literally forgot that Luna existed. She was shoehorned in during the animation phase (which made for a good joke).
The Crystal Empire - Not present. Sitting it out to test Twilight.
Magical Mystery Cure - Not present. Sitting it out to test Twilight again.
Princess Twilight Sparkle - Not present. Captured by vines.
Twilight's Kingdom - Out of commission. Acting as a decoy to mislead Tirek.
The Cutie Map - Not present. Not really anything worthy of her time.
Doesn't strike me as that bad of a track record to be honest. The vines is clearly the dumbest as far as I'm concerned, but her absence in the first two aren't particularly hot either, and both of those occurred under Lauren Faust.
IMO, the Meghan side of the show is where Celestia really loses her shine. She doesn't have nearly enough gravitas or cleverness or depth. Luna might as well not exist (aside from the dream episodes, which were a neat new trick but are getting old), and it was a bad idea for Hasbro to promote Cadence to their level. Lauren apparently refused and lost, and Meghan apparently can't write "Princess Cadence" worth a darn, so Hasbro really should've let Lauren have her way. Fewer "Princess" characters, and keep a writer who knows how to handle the ones they already have.
Meghan has been abusing the "Twilight is Celestia's successor in-training" catch all, which Lauren created.
I think it's clear by now that Lauren was better at handling Celestia, at minimum (there are probably other ways in which Lauren was better). Which isn't exactly an insult to Meghan, because Lauren Faust is practically an industry legend. And I'm sure Meghan has her own strengths.
But Hasbro earned
slightly more money in 2012 by sticking wings on pink toys and driving away Lauren Faust, and that's all that really matters.