@aerion111  
But nothing stops someone like a savant from pointing out a possibly overlooked detail, or giving an alternative idea.

@Power-Up&Sky-Blue  
Night stitch is a tailor by trade, though.  
Fluttershy knows surprisingly much about sewing, yes, but it’s not like her cutie-mark is tailoring.  
Not even Rarity is really an especially skilled tailor; She can make clothes well enough, but she’s known for her ideas more than her skill with the needle (and then there’s the gem thing)  
Night Stitch, while she specializes in things that help ponies sleep, does make a living apparently directly off her skill with the needle (most questions seem to already know what they want, so she designs rather few things herself)  
She’s also more versatile; From plushies to clothes to changeling-costumes (as in, costumes for changelings. Though she might have made the other kind too)
 
In short: No, if Night Stitch says ‘A proper costume would be too heavy’, I don’t think Fluttershy could make a light version (and, as mentioned, the version she did make was obvious in proper lighting)

@aerion111  
But her ‘freaky knowledge of sewing’ as Dashie calls it could probably work around that issue.

@Power-Up&Sky-Blue  

 
She was in a Vampire Fruit Bat Pony costume, though.  
And the wings were more obviously fake when she came out of the shadows; A more convincing costume might be heavier.

I think Fluttershy might know this sort of thing better.  
She could still fly with her covered wings.