@Background Pony #4313  
I became a brony in the first season, so I remember it starting, because of that episode though, and fans noticed Derpy more then the others.
 
The first Equestria Girls movie also featured her holding a muffin.

I don’t think it’s the same any of those other examples. Yes, the fans exaggerated one thing a character said or did but in this case it was a group shot of characters with nothing special about her reaction compared to the others.
 
The decision to have her love muffins was clearly a fan reference. However it was subtly referenced before Slice of Life. There’s an episode where she has a saddlebag with an image of a muffin on it.

@Kaiser Umbra  
Sonata loving tacos was only mentioned once in “Rainbow Rocks,” (“It’s Taco Tuesday!”) and fans ran with that too.
 
And gamer pony Button Mash, which NH already mentioned below.
 
@northern haste  
Yeah, but merch’ called her “Muffins” before that, or “I Heart Derpy face.”

@Beau Skunky  
My point is that was the only time she showed interest in muffins until the fan service episode

@Beau Skunky  
Any other examples?

@northern haste  
Her mouth moved when she said it, so the fandom took that, and ran with it. In fact, alot of memes are somewhat based off something brief, or obscure from the show.
 
Though, some like LongGlimmer just come out of nowhere.  

@Beau Skunky  
the thing is just saying the word muffins once does not make her the muffin mare(really loving muffins) it was a fandoms that did that it might not even have been her who said it.  
it’s the same thing was that gaming pony we saw in season 2 release time playing games once in the fandom turn that into his personality

@northern haste  
Actually, it kinda did, because the whole “Derpy likes muffins” gag/meme got started, because in an early episode, (“Applebuck Season,” I believe) when a gang of BG ponies including Derpy are admiring the tray of muffins, Derpy can faintly be heard saying, “Muuuuuffins!”
 
 
(Sorry, couldn’t find a video clip on this site. Though, it’s funny to look back in the old 7/8 year old comments of that image, and seeing people being butthurt about “Derpy liking muffins.” While nowadays, it’s perfectly accepted as canon.)

@Beau Skunky  
It is kind of funny howhouse some balance products that they pretty much just lost one piece of fan service when muffins is still technically a fan service name. Keep in mind it wasn’t the show that made her the Muffin Mare

@Background Pony #4313  
I was too, but I didn’t mind Derpy honestly. To me, she can be both, like Dr. Eggman/Robotnik, or King Bowser/Koopa.

@Beau Skunky
 
There were some fans really attached to the name “Ditzy Doo” because of the one liner in Winter Wrap Up.

@Background Pony #4313  
We don’t know for sure if they were trolls just doing it for lulz, but I recall during Derpygate, some people were unironically offended by the name, which is what lead to the Derpygate & “Save Derpy” situation.

@northern haste
 
Hasbro was sent about 12 emails by trolls pretending to be “concerned parents” that were offended by the word “Derpy” and her portrayal. Even though they got like hundreds of emails complimenting the scene, they listened to the trolls and censored it.

@northern haste  
That’s it?

@Kaiser Umbra  
Well the credits in the show call her muffins

@northern haste  
So what is it officially?

@Beau Skunky
 
@Kaiser Umbra  
I don’t think it’s a legal reason but more PC reason as apparently the word is a word for retarded.  
Also Derby was only called that once as quick fan service in a somewhat average episode and that version isn’t even sold anymore

@Beau Skunky  
Why can’t they use Derpy?

@Kaiser Umbra  
They aren’t allowed to use “Derpy” for some reason.
 
I think they should’ve used “Bubbles” instead of “Muffins” as her alternative.

Just muffin? They called her Derpy in the show! When I noticed this at 5below, I was outraged!

aaaaaaa I NEED IT!!!

I love the crazed pony face on the packaging.

I hope we see a derpy short.