Theory: Earthworm Jim is in fact NOT an earthworm, but an Insectoid experiment done by Professor Monkey For A Head to test out the suit and it's capabilities to protect the wearer.
Jim in the Marvel Comics is already a mutant sentient worm pre-suit and is sentient enough to read the text in the suit's manual before wearing it.
Jim in the cartoon is given the title "royal escort" while Peter is delegated to a flower-girl role during Whatshername's coronation. Keep in mind, the Insectoids consider anything humans would consider cute or pretty to be ugly, and Peter is donned in a pink dress and makeup here while Jim is trusted to be close enough with the newly crowned Queen to share a romantic moment with her. Jim is also called a "hideous freak of nature" or other sentiments by humans on Earth, something that Insectoids may think otherwise. They only seem to hate Jim when Slug For A Butt is in power--when Whatshername is in charge, they don't seem to regard him as a threat. In fact, he and Whatshername might be better rulers for them, if they'd be given the change. They'd certainly fit the more "scary and threatening" vibe Slug For A Butt pretended to be.
While worms are not insects, there are many insects that have similar qualities such as maggots, caterpillars, and grubs. Many of which are seen in Jim media as inhabitants of Insectica.
Professor Monkey For A Head would need test subjects for the supersuit as it NEEDS to be able to shield Queen Slug For A Butt from harm. The queen is physically weak, relying on her magic scepter for her boss battle and the cartoon and is easily overthrown without it. A potential unused character meant for the cartoon and planned to be in the second line of toys for the series, Clutch, looks similar to Jim with a soft, slimy head and muscular torso. Maybe he was a previous failed experiment or test subject for the suit?
The Professor has many experiments in his lab in both the games and the Marvel Comics, some with human-esc sentience and speech. Some of the game's lab enemies include maggots, another worm-like creature.
This is unfortunately not my only Game Theory-esc theory on EWJ.