To dream that someone is being mean to you suggests that you are being too hard on yourself. You need to give yourself a break. Alternatively, the dream may be a metaphor that you are looking for “meaning” or significance to some life issue.
To dream that you are uncharacteristically mean to someone represents some negative feelings that you are not expressing in your waking life. Instead they are manifesting in your dream where you are more comfortable in expressing how you really feel. Dreams provide a safe outlet.