You cannot determine the true nature, quality, or character of something or someone based solely on outward appearance.
While the sentiment is ancient, this exact phrase emerged in the mid-20th century as book publishing became more commercial and covers more decorative. Earlier versions included 'don't judge by appearances' and similar phrases, but the book metaphor stuck because it's so visually concrete and universally understood.
This phrase became popular just as book marketing was becoming more sophisticated, with publishers using increasingly flashy covers to sell books. The irony is that the phrase emerged precisely when book covers were becoming more important for sales, making it a kind of pushback against commercial publishing practices.
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