Michael Mabbott’s “mockumentary” about a fictional early-1970s country-rock star (modeled on Gram Parsons) suffers from the same stumbles that most such movies do—the “interview” footage often sounds scripted, and the “vintage” film doesn’t look very vintage. But a smart screenplay, an appealing performance from Matt Murphy (as the title character) and funny turns from Kris Kristofferson and a pricelessly droll Merle Haggard make Terrifico a hoot nonetheless.