Yes we're all still creating dungeons manually.
Yes there should be a tool to make it easier, which would enable a great many more people to contribute.
The only saving grace we have is the dev team took the time to separate dungeon.dat out into separate .dat files to make things at least a little easier for players to mod.
If we were hypothetically going to have a dungeon modder, for the UI I think something simple like the Binding of Isaac room editor
would work just fine. Similar to Isaac we'd be using the editor to create rooms that the game stitches together procedurally, but there's a few points of difference between Isaac and IVAN's level generator that we'd need to account for. For example, Isaac doesn't feature hallways and every room is pre-defined, whereas IVAN has both random and set room types.
You'd also need to define whether an object you placed in the room is positioned exactly where you've placed it, or whether it can appear on any free tile, plus the spawn chance (if any).