It's hard to come up with a word that works all the time, because much of this depends on your perspective.
Some might call this person shallow. They think this person pays too much money for a name brand – particularly if they don't have a lot of disposable income. They might view this person as a sucker for a slick advertising campaign, and as a person who succumbs to peer pressure.
shallow (adj.) Concerned mainly with superficial matters.
However, people who enjoy trendy fashions might call this same person sophisticated. They regard this person as someone who has good tastes in fashion, as someone who likes to wear well-made, stylish accessories.
sophisticated (adj) Having obtained worldly experience; cosmopolitan.
Your question says this, which makes me think that maybe you're looking for the former term:
I mean to like external looks more than internal value
but not everyone sees this emphasis on the external as a negative thing.
Another phrase that could work is material girl, made popular by a Madonna song by the same name:
material girl (n.) a woman or girl mainly concerned with material things
Material girl could be regarded as a trendy way to call a woman materialistic, which means she has a preoccupation with worldly goods, such as expensive clothes and fancy cars.