FXX’s new comedy “Dave” exists in thrall to the phallus. Its protagonist (played by the rapper Dave “Lil Dicky” Burd in a sort of takeoff on his career and persona) is both obsessed by and ashamed of his anatomy, about whose particularities we come to learn in granular detail as the show’s early going wears on, up to and including an episode about the maneuvers undertaken to keep his girlfriend (Taylor Misiak) from learning about what his genitals really look like. This, more than Dave’s attempts to convert his somewhat myopic self-regard into a rap-world persona or his personal life — including fitfully sketched pals — seems at times to be the show’s true subject. Which draws an interesting, and unfortunate, comparison. When Donald Glover made an FX show about characters trying to break into the music industry, it was “Atlanta,” among the most expansive, richly imaginative shows of the century so far. When Lil Dicky does it, it’s a show whose breaks from flatly telling us about his character’s private parts tend to follow a linear trajectory: Lil Dicky, a ditherer with more zeal for fame than true creative ambition, ends up trying something, it goes viral, everyone loves it.