What is the difference between aphantasia and a bad imagination?
They are not the same thing. A bad imagination is a struggle to come up with ideas. Aphantasia is simply the absence of an internal image. A person can have a dark mental screen and still be relentlessly imaginative.
Creativity and visualization are not the same thing. Visualization is just one road to a creative outcome. Aphantasia removes that one route without closing the destination.
People with aphantasia think in facts, patterns, concepts, and systems. They build ideas rather than picturing them. Some of the most inventive people in science, animation, and writing have done exactly this. Their creative process is often one of discovery. They find the work on the paper or in the clay as they make it, rather than copying a finished image from their head.
The confusion runs deep because almost all of our language for imagination is visual. Phrases like "picture this" or "see what I mean" make it sound like a mind's eye is required for a working imagination. It simply is not.
The brilliance of a thought is not found in the image it produces. Thinking that aphantasia equals a "bad imagination" does real damage to how people see themselves. The intelligence and the ideas are completely intact; they just land without a picture.