Mindfulness is about achieving a state of awareness and focus on the present moment and your feelings within the moment. If you are not being mindful, that does not mean you are dissociating. We are rarely mindful in our everyday lives, when we are focusing on tasks or other things but that doesn't constitute dissociation.
Dissociation is about disconnection from our surroundings or parts of ourselves. I have only recently begun to recognise my experience of dissociation; my mild experience includes my vision going white, not hearing or understanding what is being said and confusion about what is real or what has occurred. More severe episodes can include a sense of separation from self or multiple personality disorders. It's a psychological phenomenon and not to be confused with not being mindful (although mindfulness techniques may help as grounding exercises to prevent dissociation).