Yeah, it's another pun. You got a problem with that, tough guy?
When the Martian Manhunter first showed on good ol' Planet Earth, he introduced himself as "J'onn J'onzz". Get it? John Jones?
Ha ha ha. Now you understand Fred's joke in this strip. J'onn J'onzz, Manhunter from Mars, meet J'onn S'mithh, Manhunter from Neptune. Pleased to meetcha.
Presumably, the Martian Manhunter was created as an answer to all of the little-green-men-from-Mars invasion stories that were popular in the fifties. Here, we have a BIG-green-man-from-Mars, transported unwillingly to Earth, deciding to adopt a secret identity and fight crime. What a nice guy.
He's no slouch in the powers department, either, what with super-strength, flight, telepathy, shape-shifting, eyebeams, and near-invulnerability. By 'near', I mean that you could take him out with a Zippo lighter. Every hero has to have an Achilles heel, and his just happens to be fire. Personally, I see it as an opportunity for a joint DC/National Park Service publication: 'The Martian Manhunter and Smokey the Bear Say: Only You Can Prevent Forest Fires!'
All joking aside, the Manhunter from Mars is a significant character in the history of the comics industry. DC's introduction of the Manhunter is generally considered to mark the beginning of the Silver Age of Comics, a period which saw the super-hero genre pull itself out of its long decline and back to prominence. DC used back-up features and anthology titles such as 'Showcase' to launch these new concepts and characters, and many of the new - and occasionally reinvented - super-heroes quickly earned their own titles.
The Manhunter was even a charter member of the Justice League, joining DC's other super-hero stalwarts in the flagship team title. Now, that has to look good on the resume, even if he does lack fashion sense. C'mon, big guy - suspenders without a shirt underneath just doesn't work!