We expect every app developer to suddenly become an expert in efficiently producing docker layers and not just lazily use an Ubuntu desktop image?

I expect anyone distributing their app via Docker to learn how to use their tooling properly, yes?

Agreed, there are many things to get right about using Docker. But if you're just deploying an app to a somewhere, does it really matter if the image is a bit bloated (within reason)?

Probably because container authors learned from Docker's getting started video, which demonstrates a 1GB "hello world" image.