Tutu dresses are a phenomenon new to me. I'm pretty sure these are part of the current princess meme (again, maybe I mean trope).
These dresses fall somewhere between a t-shirt and ball gown - stretchy, pulled over the head, but embellished by chiffon and lace.
With a sufficiently gullible three year old, a granny could claim one of these is a princess costume. And a sufficiently unscrupulous granny could even claim the costume is that of a certain Arabian princess that wears blue.
Here's a photo of the back of said tutu dress in motion.
From Costco, price was right.