That's assuming her arms are still going through the arm-holes.
It's also theoretically possible that the dress is just really loose, so her arms are pushing the fabric down somewhere on the sides/back, and we just can't see because the front of the dress is able to flow up unimpeded by this and obstruct our view.
Still, good catch: I wouldn't have realized the unusual nature of that if you hadn't pointed it out.
It's a slang term celebrating a victory. I believe it gets its origins from the term "to score", as in: To acquire points (I think specifically it gets its origins from the tendency of people to yell out "SCORE!" in sports just after... well, scoring). In this case, though, it just refers to my happiness to see yet another magically animated clothes work, in this case coming complete with some adorable Pinky Pie fanservice <3.