"Don't judge a book by its cover."
This is an English proverb that means "don't set your mind unless you check the content",but there is another meaning that "don't decide a person by its looks."
I think this meaning has two sides.
For example, though if you think it is difficult to get along with some people at first, they can be your good friends by knowing their characteristics.
On this example, this proverb encourages you to have wide view on people.
On the other hand, sometimes this proverb tells people to become more suspicious.
For instance, you may be deceived by someone if you decide the humanity by its looks.
Good looks do not always mean the person is actually good.
Although it is easy to understand this proverb's meaning and this term is often used in Japanese too, I think how you receive this is person to person.