Considering the Dragon Quest Monsters games on the DS are in full 3D (as far as I've seen, anyway), the only plausible tools would be for changing the monsters within a certain area and changing the text. I don't think there would ever be a functional tool to make custom areas.
At best, I guess someone could go learn hacking and try to edit the landscape themself by messing with the ROM, but that might take way too long to be worth it and you probably need source code for it anyway.