This term comes from the fact that these people follow the popular media like mice follow the dominant mouse in a cage or in a mousehole.
Often old enough to think for themselves, but are often too lazy or too obsessed with conformity and being liked to do so.
See also follower and conformist.
A&R guy: But don't you think we should sign someone with talent who maybe isn't so visually exciting?
Record company executive: Nonsense Tom, these kids are media mice. They'll buy whatever MTV and BET tell them to. Put a few of these videos and TV and, bam we're rich!