Depends on the game. C2 doesn't necessarily provide what you need to build a decent level editor. Some examples...
-Can't retrieve instance variable name or index
-Can't set instance variables by expression or index
-Can't create objects by name
-No menu bar
-Very few windows controls
-Tilemap object is designed only for edit time
-Editor has to be contained in the project it is used for if you want to use the same objects and all, which can be quite the problem.
Which kind of sucks because if you want to build a custom editor for, say, a metroidvania or zelda-style game, you're pretty much screwed. C2's layout editor is fantastic for level-based games though.