~Artwork and technique: you did a great job on both Celestia and Applejack! It's easy to see that in the third and fourth panel it's filly AJ, though you only drew her a bit smaller and gave her round eyes. That does need some training, I assume.
~Use of colours and text: nice how you gave Celestia and AJ both different text styles. It.. 'adds' something to the comic. Sort of a final touch.
The colours are great, the background colours fit perfectly with the comic and hay, you even got the colours of Celestia's mane right!
Only thing I can think of that could-have-been-better are the backgrounds. It's kinda simple. But it also fits with the comic, so it's not a big deal.
~The story itself: very nice. This is something you don't think about every day, and Celestia+AJ-comics are rare.
The fourth panel really made me laugh out loud, it's also something I imagine Applejack doing in the show.
The way you made AJ speak is also quite fitting. "Land sakes! You're HUUUUUUGE!"
I'd say a 5/5, really. It's been a while since I saw a comic this awesome.
Me: Pfft... no.... must....not.....laugh..... PFFT HAHAHAHAHHAHAHHAHAHAHA
:33< i have asperger and i am very honest (sometimes too honest according to my mom)
which fat joke would that be, you ask?
Fillyjack: "And WIIIIIIIDDDEEEE!"
"When you get to be my age."
Sadly that'll never happen. ...unless... >.>
Added to headcanon: Lil' Applejack would loudly exclaim her honest thoughts on everything. She didn't get tact figured out until she went to stay with the Oranges. Granny Smith actually fed her the idea about going to Manehattan in a way that AJ thought it was her own idea, so she'd go pick up some manners.