A Sign of the Times, a new jukebox romp featuring musical riffs and cultural rifts from the 1960s, is full of statements. It has something to say about civil rights, women’s liberation, Vietnam, the course of true love and the influence of Pop Art. But this York Theatre Company production also leaves behind some nagging questions. Can a musical be “woke” when its book is tired? Can stock characters find believable ways to bond? Was Petula Clark right that things will be great when you’re downtown?