To me they look like giant shape-less smocks with a variety of sleeve cuts and bow types. For example, Momoko has a big bow and poofy princess sleeves, whereas Chinami has a little tie-looking bow and what appear to be kimono-like sleeves. (Terrible) example below:
I was going to say.. reminds me of a mix between an artists frock and Sister Act. I like the covers. I love that Ryuusei Boy cover, though.. I forgot how amazing that looked.