Why is this considered a bad thing?

"Vanity

1: inflated pride in oneself or one's appearance : CONCEIT

2: something that is vain, empty, or valueless"

The value of an individual is in what they give to others. You call him your friend. Is he good and considerate of you? Are your feelings as important as his own?

Because he's pouring a lot of energy and money that could probably be better used elsewhere into something as superficial as his appearance. Now he may be doing this to monetize some persona and in that case perhaps it's a reasonable investment in a "career" (or at least a fast grab at some cash and clout). But to people with loftier goals and ambitions this kind of thing often comes off as hollow. 

I guess cause he likes showing off and bragging which most people dislike. It makes him comes of as spoiled.

It isn't necessarily a bad thing. It just shows he's more interested in himself than in what people think of him. The Bible at Proverbs 22:4 reveals "The result of humility and the fear of Jehovah Is riches and glory and life."

The Bible says to not wear expensive clothing.  It also says not to wear a tattoo.  It says to not be dishonest.  (Plastic surgery to alter your appearance to look like something else other than God intended is not honest.)  The Bible says not to be lascivious and to be humble.  God gave the world the Bible to guide us in life so that it might go well with those that obey it.