Also, by the end, he just knows that she’s “a being from another dimension”; He doesn’t have any idea of what she really is, and the only one who does isn’t likely to just blurt out what life is like back home.

@TitaniumCupcake  
Ah; you forget that you, YOURSELF, also look that way. It’s not just “looking different”, they look the same as you, NOW, look, and you have come to that realization by this point. In fact, you put together a musical number while walking into stuff.
 
Also, your assistant is now a tablet-phone assistant-program. I know Spike’s a dog, but if you’re gonna change the species, I’m making Spike the Office Assistant, dammit.
 

 
I will call him Clippy, and he will be mine… and he will be my Clippy.
 
Anyways, I get the feeling you don’t quite understand that one’s shallow nature goes a bit beyond looks. In fact, Flash later has statistics on his side.
 
But at the start, she doesn’t develop an IMMEDIATE crush. It starts off more as a thing where she keeps bumping into him and it grows. At the start, it’s just “Someone helping”. From that point, she has a positive opinion of him. And keep in mind, it’s ONLY of him. There’s absolutely no one else she likes when she first sees him, because everyone else sees her as being “weird”.
 
Also, you’re not allowed to dictate gender-specific looks that apply; ALL creatures would look that way in an identical situation, not just females.
 
Also, by the end, he just knows that she’s “a being from another dimension”; He doesn’t have any idea of what she really is, and the only one who does isn’t likely to just blurt out what life is like back home.
 
All in all, I can perform enough explaining to justify the reaction that the writers were going for, with how much time and funding they likely had to work with. “We need a fast relationship built between them. Skip dates, skip dinner, we need a series of events occurring where he takes control of a series of bleak situations for her.”:
 

She falls down? Help her up.

Happens again? Help her again.

She’s trying to get people’s support with song? Play an instrument to show you’re on her side (also, playing riffs that fit with the song, while ridiculous, would mean to establish a “connection” between them; like-mindedness.).

Is she being interrogated by an authority figure? Save her with evidence and claim it was no problem.

Is she dancing at a prom and looking stupid? Join her.

She gets over the fact of them being strangely-shaped creatures fairly fast, since she knows time is of the essence and just accepts the “norm”. She sees Flash for his actions (being the SOLE helpful creature at the time certainly helps) rather than for his appearance.

Ok, example

You are a human, have been human your whole life. You already know what you are attracted to in both personality and looks. Suddenly one day you are sent to another world and have to spend a few days there. Your form changes to fit the new world, but you retain your human mind.

on your first day there, you meet one of their females and she treats you nice, not knowing you are from another dimension.

After a day or 2 you get into trouble, framed by someone, and she comes to your aid. This is what she looks like, and all females of this world look like this.

Because she was nice to you, are you automatically infatuated with her. Remember according to your example, Time is of the essence and you have gotten used to their look in that short period of time.

My example being, that it takes more time than what Twilight had available to overcome the difference for attraction. Yes you can eventually overcome the physical difference and learn to love the person within. But in this case it is only a day or 2, and crushes are notorious for being mainly physical attractions.

Flash has no problem crushing on twilight, in his world she looks human. Sure he knows she is actually a pony now, but is assured that she will look human any time she comes back.

Twilight does not have that luxury, she has a pony mind, and has lived all of her life learning what she looks for in a Stallion/Mare or both, in a physical and personality way. I am simply stating that no matter how nice you are treated, or if someone helps you out of a jam, it would be a lot to overcome that quickly.