'You don't judge a book by it's cover' is a cliche. Cliches can reflect truth, but usually they are the sign of a lazy writer, and it is almost always useful to assume the cliche is false. In the case of people the cliche is poor advice in both the literal and metaphoric sense.
In the literal case, book covers are carefully crafted by professional book designers. These people work very hard to create a cover which will attract the audience which the author, publisher, and designers believe will want to read that book.
And at the metaphorical level, especially with people, we all make many choices about how we are going to present ourselves.
To exclusively judge a book, or a person, by their 'cover' is short sighted. But to ignore the cover is to intentionally ignore key elements of design and self presentation.
Or am I guilty of 'judging a book by it's cover?'
I think the real point is to not make judgments about people based on things which are not relevant. And especially to not make pejorative judgments based on things which are outside of our ability to control.
Frank Zappa said"Everybody in this room is wearing a uniform, don't kid yourself." So should we ignore the uniforms we each choose? Or just consider that any particular uniform or presentation provides information, but that it is not all of the information?