They really hit the nail on the head with 'you can't judge a book by its cover.'
Literal
'People [topic-は] appearance [by-に] not-depend' [quotative-とは] well said thing [copula-ですね].
人は見かけによらない 'people don't match their appearance' — a proverb equivalent to 'you can't judge a book by its cover.' よく言ったものですね is the polite version of ~とはよく言ったものだ, expressing admiration for the proverb's truth.