Johann's behavior is like Boris Karloff's in the 1931 film version of Frankenstein (1931), complete with a fear of fire.
The dual role of identical cousins or siblings, with contrasting personalities (Herman Munster / Johann), was a common plot device and special effects' technique (using split-screen) in 1960's sitcoms. It was used previously on Knock Wood, Here Comes Charlie (1964), and also for cousins Samantha Stephens / Serena in Bewitched (1964), Carlos Ramirez / Luis Ramirez in The Convent Gets the Business (1969) and Jeannie / evil fraternal twin sister (also named Jeannie) in I Dream of Jeannie (1965).
Inside joke: the cartoon Johann is watching on the Munsters' TV is a clip from Calvin and the Colonel (1961)(1961); Joe Connelly and Bob Mosher - producers of "The Munsters" - were also producers of that show.
Much like the blind hermit (played by O. P. Heggie) did for the Monster in The Bride of Frankenstein (1935), Grandpa gives Johann reading lessons.