@Princess Trixie vi Equestria
It’s a bit more complex than that.
Ditzy Doo was mentioned by name in “Winter Wrap-Up”: she was the pegasus who was supposed to bring back the migratory birds, but messed up and flew north instead of south. By that point, fanon had largely decided that Derpy was a lovable klutz, so some fans took this episode to mean that Ditzy was Derpy’s real name in canon. Someone even asked Faust about that, and the reply was something to the effect of “That wasn’t our original plan, but she can be Ditzy if you want her to.” So the name Ditzy Doo never eclipsed Derpy Hooves, but definitely became the most popular alternative.
Then “The Last Roundup” happened. The original script by Amy Keating Rogers used Ditzy Doo as the name; it was someone higher up who told her to use the name Derpy as a nod to the fans. When that exploded in their face, Hasbro backpedaled, in most cases by going out of their way to avoid mentioning her by any name. Hence, when “The Last Roundup” was edited, they removed the name “Derpy” but didn’t give her a new name—and the comics, the trading card game, and the Gameloft mobile game all featured Derpy without naming her. There were a few merch-related cases where Hasbro tried to insist that Ditzy Doo was officially her name (the wiki mentions a Hot Topic hoodie and a conversation between Hasbro and a Gaia Online artist) but those were definitely a minority.
Hasbro’s current policy, as far as I can tell, is that a lot of employees internally refer to her as Derpy, but official sources avoid mentioning her name as much as possible—and if they have to use a name, then she’s “Muffins”.
Ironically, the “Ditzy Doo” fanon name was the real casualty of Derpygate. Since “Derpy” was now connected to the character in an episode of the show—even if it was edited shortly after, since so many fans insist the original is the “real” version—which is more than we could say for Ditzy Doo. If Hasbro had
really committed to Ditzy and outright mentioned the name in a later episode, it could have survived—like Sweetie Drops caught on as an alternative to Bon Bon, following “Slice of Life”. But Hasbro was wishy-washy and evasive about the issue, so Derpy gained more fans while Ditzy faded away.
So, agreed with
@mav that “Ditzy Doo” should just be an alias for “Derpy Hooves”, and with
@EPPony that there should be a “Ditzy Doo (Fallout Equestria)” tag for her FO:E counterpart.