OP, not the comments. <.<

Na… That’s some horrible romaji. XD

@Beatfox  
And I just now discovered there’s a Japanese version of the MLP:FiM Wiki. Sweet!
 
Continuing from there with the CMC and other princesses (and Trixie):
 
Apple Bloom = アップル・ブルーム (Appuru Buruumu)  
Scootaloo = スクータルー (Sukuutaruu)  
Sweetie Belle = スウィーティ・ベル (Suwiiti Beru)
 
Princess Luna = プリンセス・ルナ (Purinsesu Runa)  
Princess Cadance = プリンセス・ケイデンス (Purinsesu Keidensu)
 
Trixie = トリクシィ/トリクシー (Torikushii [2 alternate spellings that romanize the same])

@The Twelfth Doctor Whoof  
You did very well. Here’s the official transliterations for the Mane 6 + Celestia from the JA Wikipedia, along with their corresponding romanizations:
 
Twilight Sparkle = トワイライトスパークル (Towairaito Supaakuru)  
Applejack = アップルジャック (Appurujakku)  
Rarity = ラリティ (Rariti)  
Pinkie Pie = ピンキーパイ (Pinkii Pai)  
Fluttershy = フラッターシャイ (Furattaashai)  
Rainbow Dash = レインボーダッシュ (Reinbō Dasshu)  
Princess Celestia = プリンセスセレスティア (Purinsesu Seresutia)

Twittlelit Spickle.
 
That is her proper name, get it right!

daisuke furutaasi-chan.

Tweelight Spackle  
Pinkie Pee  
Fulutarsi  
Applayjaku  
Rawrity  
Rainbow Dasu  
Triksuhi  
Scotaloo  
Switi Belle  
Apple Blum  
Princess Ka-dance  
Princess Celestia  
Princess Luna

Shouldn’t the Princesses go by “Hime”?

imposhiburu!!!

Apple Bloom has way too many “U”s.

Actually, many of these “Romanizations” are flat-out wrong.  
_{Please correct me if I’ve screwed any of these up. This has taken a really damn long time.}
 
Twilight Sparkle = トワイライト・スパークル  
Applejack = アップルジャク (or アッペルジャク if you want an “e” in there)  
Pinkie Pie = ピンキ・パイ  
Fluttershy = フラタシャイ (or, more “correct” but worse for pronunciation, フラッテルシャイ)  
Rainbow Dash = レインポー・ダッシュ  
Rarity = ラリティ
 
Apple Bloom = アッペル・ブルーム (again they could substitute プ for ペ)  
Scootaloo = スクータルー  
Sweetie Belle = スイティ・ベル (or, archaically, スヰティ・ベル; I like the archaic kana)
 
_{(these next three assume “princess” is translated; I don’t want to hunt down the proper kanji, so don’t ask)}  
Princess Celestia = セレスティアひめ  
Princess Luna = ルナひめ  
Princess Cadence = ミ・アモレ・カデンザひめ (ケイダンス)

Forgot one: “Derpuru-chan!”

Runa-hime is best hime.

Runa  
Because f(yay)k L

I don’t get Trixie and Scoot’s names.

These names sound suspicously Engrish. Especially “Purinsesu Runa”.

pinkies just short two letters

“Runa” lol of course.

I like how Rarity’s name hardly changes.

basically is the same names but with different spelling, pointless pic.