No. Mirror muscles are the same as beach muscles. The muscles you see in the mirror. Shoulders, biceps, chest, abs, quads.
Im guilty of neglecting the mirror muscles as well. In highschool i was the typical bench, biceps, abs, calves guy, which led to some interesting imbalances.
Then when I started training more seriously, and found T-Nation, i pulled a Costanza and did the opposite. I think this was a great choice, but I took things too far, and am now trying to play catch up with my mirror muscles.
The thing with T-Nation, is that most of the articles are catered to average joes, weekend warriors, and athletes. For these people, developing a strong posterior chain is a very good idea, and doing something like 60:40 posterior to anterior is a good idea. But for someone interested in bodybuilding, it doesn't make much sense to neglect the mirror muscles.
For the first group, a bigger chest and biceps might be nice, but isn't really that important, while a posterior chain is definately a priority. And for a bodybuilder, a strong posterior chain is beneficial, but not as necessary as a big chest, biceps and shoulders.