here's my picks for the mane six and then some (try to ignore context, some have that problem :( ) :

Rainbow Dash: I'm awesome (by Spose)

Applejack: Home (Phillip Phillips)

Pinkie Pie: Dance til your Dead (for some reason, I can't find the person who made this song. just search "Dance Til Your Dead" on Youtube)

Fluttershy: Become the Wind (Adriasaurus edition)

Rarity: Pass into the Ass (by Arkadou)

Twilight Sparkle: 7 Years (Lucas Graham)

Vinyl Scratch: You Broke it Down on the Wrong Beat (by EreanMovies)

Chrysalis: Want You Gone (Portal 2 Soundtrack)

Starlight Glimmer: How Bad Can I Be? (Lorax Soundtrack)

Celestia: Everything is Alright (To the Moon Soundtrack. that is both coincidental and hilarious, but also saddening, like Celestia!)

Luna: Here Comes a Thought (Steven Universe)

Discord: Build Our Machine (DAGames)

Pinkamena Diane Pie: I Can't Fix You (The Living Tombstone)

I might add more later ;)

Now I wasn't expecting you to do secondary characters and villians too. Nicely done. I'll have to check them out.

Fluttershout: let the bodies hit the floor (Drowning Pool)

Tirek/Sombra: Be Prepared (Lion King Soundtrack)

Trixie: Confident (Demi Lovato)

Derpy: Malo Mart theme (Legend of Zelda: Twilight Princess)

OR

Derpy: Everybody's Circulation (by I'mNotGenerous)

Pinkie Clones: We are Number One (Lazy Town)

Spike: Bear Necessities (Jungle Book)

Thorax: TRAITOR!- a Mini Star Wars Song

Cutie Mark Crusaders: A-Team Theme

Nightmare Moon: Die, Motherf**ker, Die! (by Dope)

Maud Pie: Particle Man (by We Might Be Giants)

Zecora: Word Crimes (By Weird Al)

Adagio Dazzle: Obey Me (Phineas and Ferb)

Discord (again!): It's Raining Men (Living Tombstone version)

again, might add onto this!

POW! EVEN MORE!

Steven Magnet: I'm Gay (I'm Blue Cover)

Cheese Sandwhich: literally anything Weird Al wrote ;)

Thorax (again): What is Love (by Haddaway)

Trixie+Rainbow Dash duet: Anything You Can Do [I Can Do Better] (sung by Tom Wopat and Bernadette Peters)

Diamond Tiara: Mine Song (Lazy Song)

Derpy (. . .again): Nyan Cat Song

Zecora (again): Witch Doctor (Joanie Bartels)

SURPRISE!

Discorded Mane Six: No Mercy (Living Tombstone)

Daring Do: Indiana Jones Theme

or

Daring Do: Legend of Zelda Theme

Sombra: You are Dead (Total Distortion)

Cadence after giving birth to Flurry Heart: It's over, isn't it? (Steven Universe)

Big Mac: Macintosh Plus XD

Demon Sunset Shimmer: I Put a Spell on You ( Annie Lennox)

Sunset Shimmer: Borderline (Madonna)

Doctor Whooves: Back in Time (Huey Lewis and the News)

What, no daring Doo? You do derpy three times X3

2 things:
1. holy crap, I mean I have nothing against derpy, but I don't like her THAT much!
and
2.we all know Daring Do would have Indiana Jone's Theme. Search your feelings, you know its true.

I'm not saying you like derpy that much. Why you jumping to conclusions, hmm?

also, if you see any more I missed or want to hear what I think they're themes would be, don't hesitate to ask :) I'm definitely sure I missed afew

Pinkamena Diane Pie = Pinkie Pie

also, For Rainbow Dash: High Hopes; Panic! at The Disco

My theme songs for the ponies are from Super Mario Galaxy 2!

I was thinking on it for a bit and I got this so far.
Rainbow dash: bring em down by lost prophets. ( It was either that or your gonna go far kid by the offspring. )
Pinkie pie: hello by OMFG.
Applejack: good girl by Carrie Underwood ( I couldn't think I of any other song. :( )

OMFG! I love OMFG's songs!

Ok, dashie needs Your gunna go far kid

I feel like laughing but I can tell your serious XD

Applejack: Puzzle Plank Galaxy music from Super Mario Galaxy 2!
Rainbow Dash: Slider from same game
Daring Do: Slipsand Galaxy Music (same game)