Late to the game here, but that might have been intentional, even in the “heightened” state of one of the fantasy strips. Until he strongarmed the syndicate during one of his “sabbaticals,” Watterson was stuck with the same restrictions that everybody else was, namely that papers could simply elect not to run the first two panels. That meant that everybody had to “waste” the space on a title block and a throwaway gag. I could very easily imagine him being a bit petulant and making the throwaway particularly coarse, though TBH not entirely out of character for Calvin trying to play cooperatively with Susie.