I recall reading about a very successful animator who needed to sketch out every frame because he had severe aphantasia and thus had general ideas about what he

I recall reading about a very successful animator who needed to sketch out every frame because he had severe aphantasia and thus had general ideas about what he wanted things to look like but couldn't envision the actual result without a visual aid. Having an "inner picture" is helpful but neither necessary nor sufficient to be a designer. You can have a good intuition even when you're physically incapable of having "inner pictures". But yes, the tool is probably less useful if you have a vivid imagination.