It's been said already, but Robin is not a sword specialist and is well-rounded and talented but not a master at anything. She's more adept at casting spells (think of cut scenes with Chrom and last scene on the final boss) than swinging a sword.
As for the strange ledge attack and the way she rolls, think about it like this... if you were reading a book on self-defense tactics and were told to be evasive, crawling on the ground and rolling around would be techniques that would be stressed. I feel that Robin is a character that is trying new strategies even during the battle. When Robin is introduced, she is trying to relearn her skills - or rather she's trying to relearn what she's supposed to do and trying things out.
I feel that this is a pretty great design for someone like Robin, and I'd actually be a little weirded out if she swung a sword as well as any of the other swordies in the game.