I am sure that you have powers strong enough to devastate an entire army, but your sleek appearance would be a delight to little girls everywhere, so many nations would look down on you.
 
Also, Princess Luna, you are also part of a toy franchise for little girls, so princesses in general, being associated with girly toys and fairies would be seen as “cowardly” or “weak”.
 
If only you or other princesses would have been more warlike, like princes, others would think differently.
 

NASCAR fans sometimes refer to Danica Patrick as “Princess Sparkle Pony” for an insult. The term has nothing to do with MLP and was coined before the show started.

@PonyPon  
This.

Pretty pink pony princess.  
I prefer perky pony princesses though

Because all the good princesses are sexy.  
…  
…What?

I…I meant it as a compliment. Don’t hurt me!

It brings to mind insulting stereotypes that are older than you are.