In most cases, one should never judge a person by external appearances. Sometimes when I walk down the street at night, I avoid people who are acting tough and loud, but in general, I prefer to reserve judgment until I get to know someone. Judging someone by external appearance can be deceptive. Judgments based on external appearances prevent you from getting to know a person reinforce stereotypes, and are superficial and limiting.
In high school, I stayed away from students who were called the "bad students" because they dressed a certain way. I wanted nothing to do with them. I later had the chance to meet one of these "bad students" because of our mothers, we started talking and I realized that we had a lot in common. My impression of him was very different once I got to know him.
If you form an opinion of someone based on stereotypes, you risk never getting to know someone who is different. You may are be missing out on an opportunity to make a good friend. In addition, you are reinforcing that stereotype by believing it without given the person a chance. If you take the time to get to know the person, you might become friends.
Judging people by external appearance is superficial and often unfair. After all, you don't know what circumstances the person might be facing or who the person really is. Perhaps the person comes from a less fortunate family than you and cannot afford the kind of stylish clothes your friends wear. However, that does not mean the person is less intelligent or interesting than you are.
People should not be naive about new people they meet, but should take time to get to know them. External appearance often does not tell us anything about a person. Judging someone by their appearance is misleading, reinforces stereotypes, and is limiting. Doing so could prevent you from making a true friend.