If Mark Millar thinks Captain America: Civil War didn’t have a a lot of humor, someone should probably keep him away from Batman v Superman: Dawn of Justice.
Anyway, while it is true that Captain America: Civil War didn’t have as much comedy as previous Marvel movies like Guardians of the Galaxy and Ant-Man, to say the movie was missing a lot of humor is an interesting take.
(Actually, he later mentions that he didn’t care for Batman v Superman’s creative direction, but that’s beside the point.)
Plus, a story about superheroes in conflict over registering with the government has to tackled relatively seriously, no matter which angle you’re coming from.