@Turnways_Flip  
That would be so cute :3
 
I still think Scoots is a better fit for Monster Kid, though. Self-conflict, ugh~ ;p

@Ren  
All three of the CMC  
should be Temmie

@Tentauncool  
I can agree with almost all of this; really great suggestions. :D Although, I do have to interject about a couple of things. Napstablook isn’t just bland–she’s straight up shy and moody. I think if one of the Pies were to fit, it’d be Marble, not Maud. Even more passionately, I’d argue that Scootaloo would be Monster Kid. He’s adventurous, and more than anything, he idolizes Undyne (which would be Rainbow). I know Spike needs to fit in there somewhere, and if Scoots is included, the other CMCs should be, too–I’m just not sure where. Anyway, that’s my two cents; other than this, I’m pretty much in full agreement with you.
 
Oh, and you misspelled “Papyrus”.

Is that Discord as the flower? LMAO!

7/10 needs more Muffet T^T – IGN
 
Seriously though, this is super awesome! And accurate character comparisons! :D I just need more Muffet in my life. MOAR MUFFET PLS~

I was JUST thinking which Undertale character would each MLP character be. I thought nearly the EXACT same choices you made! And it looks amazing!!!

@NicoleLady  
Agreed, Toriel would be better compared with Princess Celestia.

@NicoleLady  
well that and Toriel’s theme song doesn’t involve the words “CRAWLING IN MY SKIN”

@Background Pony #43AE  
I agree with most of you on this (Aj as Sans and pinkie as Papyrus) but I personally see Toriel as more of a celestia than luna. Considering at one point Luna was a threat to the main six it would match with the time asgore took the 6 souls. Celestia has a hard time forgiving her and send her student (frisk twi) forth to make friends. I dunno that’s just how I see it.

@Background Pony #7859  
Nah, dude. Not Luna. I can see AJ as Papyrus (for the forward and earnest nature) and Pinkie as Sans (simply for the fanon 4th wall breaking), but as is, it’s perfectly fine. AJ doesn’t seek out fame or attention, nor does Sans. In fact, before you really know what Sans is capable of, he’s rather plain if a bit weird, so Luna would not fit at ALL.  
However, I see the pic’s setup working just fine as Papyrus is almost openly silly and almost air-headedly optimistic (which fits Pinkie more), while Sans is more subtle and willing to toss quips and jabs at you (which is more of AJ).  
Plus, Luna’s always felt like the sort who wanted to share love and knowledge and was willing to do what she thought was best (pretty Toriel-ish), while Celestia fits Asgore to a T (both are extremely powerful yet kind and pacifistic rulers, but are willing to take matters into their own hands/hooves for the greater good of their respective races).  
Really, this pic is excellent.

@Keith Mowz  
also why is aj sans? that makes no since.

@angrybrony  
Honestly in that regards she and celestia should be switched to asgore and toriel respectively.

luna’s far to emo to be toriel  
she can be blooky

Hmmm.. I might as well shove my ideas here. Spoilers are covered.
 
Frisk: Twilight  
Chara: Lesson Zero-kinda Twilight  
Toriel: Celestia  
Flowey: Starlight  
Asgore: Luna  
Papyrys: Limestone  
Sans: Pinkie (Because well, dumb jokes and all that 4th wall wrecking and teleporting)  
Skeleton of bad times: Pinkamena  
Undyne: RD  
Alphys: Fluttershy  
Mettaton: Rarity  
Doggo: Applejack (idk honestly?? I just don’t wanna leave good ol’ AJ out)  
Grillby: One of the dragons, maybe?  
Dogamy and Dogaressa: Shining Armor and Cadence  
Temmie: Derpy  
Aaron: Bulk Biceps  
Nabstablook: Maud  
Onionsan: Steven Magnet  
Monster kid: Spike  
Dummy: Discord  
Burgerpants: Cranky  
Bratty and Catty: Lyra and Bonbon

@Psycho Shy  
I wish I could upvote this comment

I suppose I’d better go throw in my personal opinions, eh?
 
Toriel is Fluttershy because…really, who’s better to teach you about mercy?  
Napstablook is Vinyl Scratch; not quite the same character, but I think the association of being DJs works out well there and the fact that she also happens to be a white-coated unicorn doesn’t hurt.  
Papyrus and Sans are Pinkie and Maud because they share a kind of energetic/apathetic sibling dynamic. It’s not a perfect conversion, no, but Maud is a perfect character for handing out bad times.  
Undyne is Rainbow Dash. Because she’s the heroine that never gives up.  
Dr. Alphys is Twilight. She doesn’t take things lying down like Alphys does, but it’s a socially awkward nerd. Who else could we send?  
Muffet is Applejack. I was weary of this one before, considering Muffet’s minor role, but you could repurpose her story into reuniting the Apple Family after years of being scattered all over the place.  
Mettaton is Rarity. Because fabulous celebrity.  
Asgore is Celestia, and I know everyone’s ready to jump on me here because she’s not related to Fluttershy, and I’m not sure how much I care. I think it’s safe to assume that Celestia would do anything for the sake of her nation.  
Flowey/Asriel is Luna, so the family ties with Asgore still exist. I’m not sure exactly how all the different forms would pan out (And I do acknowledge Discord would rock the Photoshop Flowey look), but God of Hyperdeath can totally just be a Nightmare Moon expy.  
Frisk is…still Frisk. Kinda defeats the point to not have it be a “stranger in a strange land” story.  
Chara is Megan. Yes, Megan Williams, from G1. Why the hell not? It could probably be worked into some backstory where Megan and Luna both died fighting Tirek, or somesuch.  
Dr. Gaster is Starswirl. Whose work does Twilight so wholeheartedly admire?  
I guess Derpy would make a pretty good Annoying Dog? I dunno.

@Background Pony #2908  
dirty friend killer
 
@NicoleLady  
Why TwiFrisk?

I suppose Applejack will call Twilight a ”Dirty possible cousin killer?“

@MarcusFenix102  
Rainbow Dash could sort of work as every character. I tried to draw her as Sans once doing the “get dunked on” thing, but I’m not enough of an artist to make the style work.

rainbow dash as undyne?
 
[Anime INTENSIFIES]

Frisk: Twilight  
Chara: Alicorn Twilight  
Toriel: Celestia  
Asgore: Sombra  
Flowey: Starlight  
Papyrus: Pinkie  
Sans: AJ  
Undyne: Rainbow Dash  
Alphys: Fluttershy  
Monster Kid: Spike  
Metaton: Rarity  
Tem: Derpy
 
That’s how I see it…

@Turnways_Flip  
Oh yes, that’s really good, you could have it follow one of her rainboom-stopping attempts, thus allowing enough changes… though you’d have to either say “canon? what canon?” or the more depressing timeline split after each attempt and a version of her was stuck in each.
 
Actually that fits better, her staying after Twilight returns to the future, then realizing she isn’t coming back (because this timeline is already set), and the implications that has on each of her other attempts - not knowing how many more other versions of her would do before Twilight wins or gives up sounds like a good base on which to build the kind of character development needed for her to completely Flowey-style crazy.

@Align  
It has come to my attention that Starlight Glimmer is almost dead-on Flowey.
 
“I’ll SAVE over your own death and replay it… over… and over… and over!”  
“Why are you being… so nice to me?”  
*Flowey ran away.
 
She doesn’t have parents though.

@Keith Mowz  
That being said, Dash as Undyne is pretty darn spot-on. It’s probably a very good thing that clouds can’t burn.