This question is similar to "do "mod-development" questions belong here".
What is our stance about game customization using built-in tools provided by the company, as opposed to customization using external tools? For example, many games come bundled with a map editor, allowing the creation of new maps or levels. Should questions regarding that map editor belong here?
Distinctions should be made.
These questions definitely belong here, in my opinion. They are about using tools provided with the game itself, and are in no way related to game development or design.
These questions usually belong here, in my opinion. Even when not facilitated by external graphical tools, they usually only involve changing data structures exposed by the game itself.
These questions definitely do not belong here. Personally I would vote close, then still give a quick answer (or rather comment?) with pointers to where to ask instead.
You can add cats to IVAN -- but you must change the game's source code. This game is closed source, so you will need to ask the developers on the game forums.
or,
You can add the tickling gun to UT3 -- but you must change the game's source code. I do not have the expertise to tell you where to change what how, but the guys at Game Development sure can. Why not ask there?
Think about what the purpose of building a map is in the game. There's two reasons it is done - as a required part of game play to get the full experience (what I'm guessing that Little Big Planet falls under), or as an accessory that creates a new experience. The latter is the only one that really needs to be in dispute.
If anything, a map editor is one of the best ways to get into basic game design matters. Yes, it's not the same scale as developing a completely new game. But what it does is introduce the matter of design-work for creating new content in a game. This new content not derived from normal play of the game, but to upgrade the current game play. Which is not really different whether the game provides standard tools for the task or not. Asking how certain elements in a map editor works or how to setup events in a map editor, is really not different than asking how certain elements in game development software work or how to setup events in game development software. The content and nature of "How can I set a trigger when someone moves a Marine into the lower right corner in StarCraft map editor" is no different than "How can I set a trigger when someone moves the player character into the lower right corner of the map in RPG Maker XP". It is not very easy to justify the former without justifying the latter - there aren't any sensible grounds.
We can field questions that result from people playing with new content - playing with user-created maps and mods. But we're not intended as a resource for creating this content. Allowing map editors really just opens the gates for all manner of game design - because there really is minimal difference in the principles behind them.
I vote that these kind of questions should not belong here.
The line between map-making and mod-making is anyway blurred for many games, and I think the questions map-makers are likely to ask are probably much closer in spirit to other questions on the game-dev site.
However, if an integral part of a game is to make customized content, and making that content is also sort of internal to the game and not a different aspect of it - for example, if you can get "high scores" for map-making - then it might be relevant after all.
Since voting on the GameDev-question indicates these questions are welcome there (and assuming it'll succeed through beta) we can redirect (and later hopefully just migrate) mapping and modding questions over to GameDev.
I addressed both modding and map editing in the mod-development question, so I thought I would add a link for a cross-reference:
