Ken Ludwig, who’s generally out to make his audiences laugh a lot and not think too hard (Lend Me a Tenor, Moon Over Buffalo, Crazy for You), strikes a more mellow and reflective tone than usual with his latest, Dear Jack, Dear Louise, at 59E59. An epistolary lark, it shares some traits with Pen Pals, still puttering away at St. Clement’s: two characters, a deepening relationship between them, lots of letters, punctuated by dialogue. Again, though, the audience doesn’t have to think too hard: Dear Jack, Dear Louise is friendly and diverting, but it sure is light.