Popularity is a hallucinogenic compound which gives us a high or takes us to a different level of pleasure. Which makes us float above this world and we feel powerful, being loved, in demand, useful, authoritarian, and someone who can turn the course. Popularity gives us the benefit of being listened to even if we snore whereas a common man's pathetic cry could not be. Popularity makes us visible like Michael Jackson.
Popularity is hyped up in the mass consciousness as something desirable, even essential. However, it is only really needed by people who cannot derive self-worth, except through the adulation they receive from others.
Those who wish to be popular often have to lower themselves to the lowest common denominator of the society in which they live.
Yes and no. Joining the "in" crowd, to be popular fits exactly what you say; that person's self worth is being defined by the group of people around them and is a sad commentary on the entire group.
On the other hand, being popular can mean you have a large circle of honest friends; people that like you, respect you and truly enjoy your company. Man is a social animal, desiring the companionship of other members of the species, and popularity in this sense of the word fits right into that built in genetic aspect of humanity.
It isn't important. People think it's important because they're too afraid of the consequences of not conforming or they think their lives will be significantly better if they do become popular. I mean, that's how it is in all the movies: "ugly" girl gets makeover, boys start to notice her. It's ridiculous, make-up can't enhance someone's personality (which, by the way, is what counts).