I agree with half of them, but the other half I would’ve switched thusly:
 
Twilight as the UK.  
It has the largest library in the world (The British Library), it has been at the forefront of science for centuries, it’s the only monarchy among these nations (which fits Twilight being a princess), and it has also always been associated with magic and wizardry (which fits her element of harmony and magical powers).
 
Fluttershy as NZ.  
It has some of the most beautiful and unique nature anywhere in the world and its isolation fits Fluttershy’s uhm.. shyness. New Zealand also ranks highly on environmental issues and has a ton of cute cuddly animals.
 
Rarity as Canada.  
It’s a kind of more sophisticated and well-mannered version of the US with a large French influence (which is something Rarity would appreciate). Canada’s and the US’s relationship also fits how Applejack and Rarity act towards each other: they may make fun of each other or have some disagreements at times, but deep down they care for each other deeply.

@Babyish and Proud  
I think Rarity is supposed to represent an upper-class American, as her accent is very specific to that group of people, especially in and around New York City. Her being a New Yorker makes more sense when you consider the fact that New York City itself is full of designer boutiques and fashion design studios. This aspect is why Rarity was shown to open her boutique in Manehattan, which is literally an MLP representation of ManHattan, NYC.

@Enmity1498  
And not British Rarity?

@Babyish and Proud  
The only one of these I will agree with is AJ being American, because she is literally a stereotypical Southerner, both with her accent and with her outfit.

Okay, a little stereotypical, I guess…