It’s a classic critical gambit to damn a show with its own words. Musicals in particular seem susceptible: Last year’s Scandalous was asking for it with a song called “How Could You?” Misbegotten revisals of Merrily We Roll Along will forever be tarred with the great Sondheim line “I liked it the way that it was.” You would therefore think it too much of a known risk for the savvy authors of the new musical version of Little Miss Sunshine to open their show with a number whose theme is the inevitability of disappointment. “Underestimate everything,” the lyric advises. “The key to contentment is to lower the bar.”