Like someone else said, a lot of the Mercies that don't deserve to be in certain rank are gone, leaving the good ones in said rank. Lately I've actually been getting good Mercies, something which was completely rare for me pre-nerf (if they got stuff done it was because of how strong she was, not because of skill). Even with the huge nerf they've managed to still be useful, which means the nerf has definitely helped the Mercies that are actually good.
Best nerf thus far, in my opinion. Comp has changed a lot, for good, since then.