RD: Oh yeah, I look good. But no one must ever know my secret.  
[Scootaloo walks in.]  
S: Hey Rainbow I- what are you doing?  
RD: Arg! Um, nothing?  
S: But why are you dressed up like that?  
RD: Alright kid, I’ll level with you, when I’m alone I…kinda like doing this, OK? In secret. This hasn’t, I dunno, shattered the image you hold of me, has it?  
S: Um, well, it’s a little weird, but if it’s what you like doing, that’s cool.  
RD: Ah, good, I thought you might freak out or something- wait what are you doing?  
[S starts dressing up too.]  
S: Hey, as I said, if you like doing it, it’s cool!  
RD: Um, I don’t know…  
[RD looks away for a bit whilst thinking about whether S doing it as well is good idea.]  
RD: Alright. It might actually be kinda fun to have some pony else to share this secret with- wait where did you go?  
[S has left the room. From outside…]  
S: Hey guys!  
RD: Wait what!? No Scootaloo, I said in secret, in secret!  
SB: Um, why are you dressed like that?  
S: I just found out that Rainbow Dash likes dressing up like this, so I’m gonna do it too!  
RD: Nooo!  
AB: Wow, really? I’ve got to tell ma sister about this!  
SB: Me too! And knowing how she likes to gossip, soon everypony in Ponyville will know about this!  
RD: CELESTIA DAMMIT SCOOTALOO!

This is what I think goes on too

Moar, MOAR TOMBOYS WITH FEMININE LOOKING!!! I love this!))

Rarity would LOVE to see this!

Rainbow Dash always dresses in style.

…Secretly of course, but as the description in the source points out, there’s a lot of evidence to support that.

I have the exact same idea when it comes to Rainbow Dash…She loves looking nice, and dressing up with Rarity…and she fucking knows it.

Rainbow Dash always dresses in style when noone is looking.