No you should really avoid relying on the cover to make a judgment. That means do not judge a person based on presumptions. I know there are many people who like to judge others based on how they dress or talk. But a wise person should do otherwise. If you see someone wearing messy, disheveled clothes, you think that person is messy and disorganized, but the opposite might well be true. You could see someone who looks intelligent, but the second the mouth opens, all of your thoughts vanish.
The problem with most people is that they are too stereotypical. They make assumptions about people based on race, gender and religion. Everyone has an opinion about somebody and that is fine. But if a lot of people all think one way, it is not good. This is what turns a group of ordinary people into a deadly mob. So, in a word, to judge a person well, you want to avoid making assumptions at the superficial level. Try talking with the person more often to get a whiff at what he or she is really made of.