Imagination is fleeting, ephemeral, jumbled, and usually linked to a specific state of mind that passes once the person has either grown bored of the imagination of achieved whatever satisfaction they wanted from it.
A deep fake image or video is persistent on your device and may leak some day, but worse is that feels real in a way mere imagination never can and it's something a person could go back to over and over, feeding into and enhancing their obsession with the non-consenting person.
I think it's a bad thing in ways that imagination and porn are not.