That theme in full: "This is 29 Acacia Road. And this is Eric, a schoolboy who leads an amazing double life. For when Eric eats a banana, an amazing transformation occurs! Eric is Bananaman! Ever-alert for the call to action." (cue morse code...)
The joke with Bananaman was that he was dense. Very, very stupid, and somewhat cowardly, but, happily, immortal. He won through by just being incredibly thick-skinned, and by having a pet crow of markedly greater intelligence. Compare and contrast with other animated superhero piss-takes, such as Inspector Gadget, Batfink and, of course, Cosgrove Hall animated-housemate Dangermouse.