Actually, I'd have to argue that Super Mario World is hacked more. Believe me, I've seen a lot more hacks of that game. It's even easier to ASM hack it than Pokemon games. There's way more ASM support for that game, as in, there's a huge ROM Map and a huge RAM Map at a certain SMW hacking website (that probably gives it away to many people).
Edit: I'd also like to point out that no one seems to have made a RAM Map or ROM Map on this site for any third generation game. There's something over at another rom hacking website for Pokemon Gold (hint: "rom hacking" isn't part of the name of the website I'm talking about).