This pose of Rainbow Dash looks like an emblem.
 

@Scrounge  
talks Rarity voice Uh. No,no,no,no,no,no,no,darling!

“Come here so I can hug you to death!”

“Look at me! Don’t I look awesome with this new body paint?!”

Hello gravewalker

She’s so adorable!

@infinita est lux Solis  
Simply put, Evil-Pie-Hater-Dash is an edgelord.
 
How, you may ask, is this related? Simple. It is from this polygonal appearance that we, the viewers, can appreciate a small but clever joke on the animators part. If we were to view this version of Rainbow Dash in three-dimensional space, she would not appear smooth and round, but instead rough and “low-poly.” Or, as the formerly mentioned joke would have it, “edgy.” She is covered in edges. Edges between the flat, geometric faces that make up Evil-Pie-Hater-Dash.
 
Or it could just what jhayarr23 said and it’s just Pinkie’s imagination. Hell if I know.

@infinita est lux Solis  
Yeah. asides to that reason, I really don’t know why they made it that way.

@jhayarr23  
true, still… kinda looks like my old MS paint drawings…

@infinita est lux Solis  
Well, i guess due to the fact that this is inside Pinkie’s imagination.

Why does evil dash look so… polygonic