As a nostalgia piece redolent of ’80s-era pop culture, “Ping Pong Summer” works better as an affectionate touchstone than fully realized movie. Written and directed by Michael Tully and filmed on location at Ocean City, Md., this summertime coming-of-age comedy possesses winsome charm and a sense for the artifacts of the age: cassettes, boomboxes, parachute pants and Jheri-curl activator.