One who desperately tries to follow the crowd or fads. Never has their own opinion and changes desperately to suit whatever is "in" at the moment. Different than a sheep who just stumbles along with the crowd because the sheepalala lives and breathes attempting to be like everyone else but usually failing miserably. Also can be called a fad junkie.
You dyed your hair pink just because Paris, Nicole, and Brittany did? What a sheepalala.