Is there a word or slang expression for girls who are preoccupied with materialistic things, such as bags, rings, necklaces and so on, and with showing such things off to others?
Girls for whom superficial appearance is more important than internal qualities.
The word "poser" comes to mind.
There are a few words and phrases that generally mean this sort of thing but usually they refer to a specific aspect. Someone who overly values worldly possessions would be "materialistic" but a materialistic person may or may not have the motivation to show off those possessions. They may simply be a hoarder or even secretive about their possessions.
Someone who likes to show off could be "vain", "pretentious" or compensating for insecurities but that may also apply to someone who pulls stunts for attention or someone who tries to "win" every conversation.
"Shallow" is another broad term that encompasses most of what you describe.
(vulgar slang alert)
fashion whore
A preachy know it all under a skin of designer clothing. Criticizes everything she considers "out of style" or "tacky". (from the Urban Dictionary)
As @J.R. says: use this term with caution.
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. Elegant, refined.
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.
The first thing that springs to mind is fashionista: a person who follows trends in the fashion industry obsessively and strives continually to adopt the latest fashions. Some see fashionista as a positive description, and some see it as somewhat negative, so context is important. Anyone who felt fashion was a somewhat silly and shallow pursuit would think of fashionista as negative.
Another term is brand-conscious: aware of popular brands and wanting to buy particular brands rather than others. This isn't really used that much in informal speech with friends. It's more likely to be used in a news article about "brand-conscious" teens needing expensive sneakers or jeans.
Materialistic seems fit
adjective
excessively concerned with physical comforts or the acquisition of wealth and material possessions, rather than with spiritual, intellectual, or cultural values.
adhering to the philosophy of materialism, a theory that regards matter as constituting the universe and all its phenomena.
See also materialism
"Fashionista" is a less insulting term for a girl - although if she's eighteen or over you should call her a woman - who buys and wears designer clothing and accessories.
It would however not be a good fit for a woman or girl who wears expensive purses or shoes but doesn't wear designer clothes, or one who doesn't buy a new wardrobe at least twice a year. A fashionista changes her wardrobe with the season, is always up-to-date, and is dressed from head to foot in the latest fashions.
Brand junkie seems to convey the meaning quite nicely (hey, it's an addiction!) without being too vulgar.
For a modern slang term, and depending on the quality / desirability / rarity of the products, she could be described as basic.
"Basic is a slang term in American popular culture used to pejoratively describe people who like popular, mainstream products or music." (source)
"Gucci Gucci, Louis Louis, Fendi Fendi, Prada / Basic bitches wear that shit so I don't even bother." - Kreayshawn
brand whore
This is about as offensive as what @Glorfindel came up with. The difference is that it implies loyalty to specific brands rather than fashion in general. There's also an opposite meaning that the person is working for that brand to raise its profile.
Some great answers have been posted, the majority of which are generally not offensive. Here are a few more slang terms that are common but provocative and insulting, and as such should never be said directly to the person they are meant to describe unless your explicit intent is to insult them. You should also only use these terms among friends in an informal environment.
Valley Girl
Invoking the stereotype of a shallow, unintelligent rich girl who is probably spray-tanned orange and has nothing better to do than drive around town in daddy's convertible buying high fashion items with daddy's credit card. The "Valley" comes from the origin of this stereotype, the Valley in southern California.Bimbo
A girl who is attractive because she's exceedingly obsessed with being perfectly groomed and adorned in high fashion and glamorous accessories. However, she has a personality that makes a puddle seem deep, as the total sum of her life goals are to sexualize and objectify herself for the purpose of being desired.
Since I feel a slight tingle of sexism being a man and writing this answer, here are some terms to describe men in the same way, some negative, just to balance the scales:
Metrosexual
Abnormal (as in above average) interest fashion for a male, overly concerned with self image.Player
A player is a male who, among other things, focuses intensely on his own image due to the the desire to appear wealthy and sexualize himself for his adventures in manipulating others, primarily people of the opposite sex. This is not a nice thing to be called, because it generally means you're sleazy and untrustworthy.GQ
GQ is the title of a popular magazine for men, fully named "Gentlemen Quarterly". This magazine is known for featuring well dressed, well groomed men. Calling someone "GQ" is generally a compliment in recognition of their taste in and use of fashion.
label whore
someone who only wears brand name clothes
further reading: http://therefinedpursuit.com/2011/10/label-whore-vs-label-lover/
Brand Loyalty
I'm surprised that nobody has come up with this one yet. I'm a guy, and we use this to refer to guys in the same situation, and I think it'd apply to girls also.
He's got Nike shoes, a Nike shirt, Nike socks, a Nike backpack, he really has brand loyalty.
You might call such a person a aficionado.
http://dictionary.reference.com/browse/aficionado
an ardent devotee; fan, enthusiast.
We often use this word to describe someone who is especially interested in something niche. You might describe this girl as a "brand aficionado".
"Label queen" can apply to both sexes, and isn't TOO insulting.
Preps? That's what we called whoever had all the top brand clothing.
I've heard the term glamour girl although its usually used with positive connotation. A stronger derogatory term would be basic bitch. A less frequently used insult I've heard someone refer to a person like that, with emphasis on how much makeup they wear is Barbie Doll. In highschool there are different flavors of the fashion obsessed: preps wear bright or casual designer clothes such as Holister, American Eagle, etc, whereas goths and emos wear dark (mostly black) clothing, makeup and hairstyles. Basic bros are guys who often dress like preps or wear excessive sportswear such as team baseball hats, jerseys and most likely pricey footwear such as Nike Jordans even when they themselves do not require sportswear or participate in sports.
I'm sure I've heard the phrase "they're very label-y" for people who choose products which flaunt designer logos very prominently. As a new coinage I don't know if spelling has standardized; I can't find it on Google, which points us, equivalently, to Urban Dictionary's "Label wh---". (I don't know how to do the hide-spoilers thing here.)
Getting into niche slang territory:
It's commonly associated with the sneakerhead and streetwear fashion circles to describe someone who is incredibly focused on the absolute latest trends. Brand display, either in actual name or iconography (logos, patterns, specific design elements), is essential in signaling how fresh and current your wardrobe is. Typically used in a pejorative manner, but can also been seen as a strong adjective without negative connotation.
