I remember that Casey said there won't be a level editor for HH because the world is generated every time the game runs. But what is the most common case in the industry for 2D games? Based on some stuff I read, I understand that a (static) level is just a mapping of sprites/entities on the screen and the mapping is stored in some kind of file. If the file is created/edited manually, that sounds like a lot of uninteresting work. Is it a common case to create a dev/internal level editor for a 2D video game? If so, what technologies are usually used (WPF? C# forms?)?

Yes, typically you create level editor. It can be any language and any framework - it really depends on studio or individual dev to choose that.

If you want something simple (but powerful enough to do a lot of things) take a look at stb_tilemap_editor.h from Sean Barrett.

Here are videos on YouTube demonstrating its capabilities:
https://www.youtube.com/watch?v=f9sdvwTBBOg
https://www.youtube.com/watch?v=lN3Js-7r6BA

There are a lot of choices. I like using OGMO (http://www.ogmoeditor.com/) and TileED (http://www.mapeditor.org/). If you're not using an integrated editor, I'd suggest doing something similar to Casey's live code reloading, check the file modification time and auto-reload every time you save or export the tilemap, it helps iteration a lot.

Frogotto has a really nice looking editor, some good stuff in there:
https://www.youtube.com/watch?v=ri614C_Buwg
And I think it's open source so you can check it out to see how they built their editor.

Thank you for your answers. I will check those links out.