Godzilla vs. Megalon isn't totally incompetent. After the underground kingdom of Seatopoia (headed by Robert Dunham in a killer toga) sends their 150-foot tall robot beetle to the surface, it attacks a dam. Megalon stands proud in regards to his upcoming destruction, then barrels though the center of the structure in a miniature effect that even makes the water convincing. That's tough.
The rest? Total incompetence. It's not one of those films you can throw squarely in the lap of the filmmakers, sort of a skeleton crew from the heyday of Godzilla offerings back in the '60s. Gone were most of the background leaders, leaving director Jun Fukuda to take the blame despite Toho's shrinking budgets.
Also lost was Haruo Nakajima, retiring from his role inside the Godzilla suit, replaced with an unknown in Shinji Takagi. Any essence of the original monster had long since been removed, turned into a doughy-eyed beast that romps playfully into battle. Maybe Godzilla considers these his weekend excursions, but Takagi doesn't even try to emote from the confines of the suit. He even walks casually across the miniature stage, arms down at his side, almost giving up.
Toho was near their breaking point too, having only a few more greenlit sequels after Megalon (before a mid-'80s revival), which showed some signs of trying. Megalon was little more than a marketing effort, situating a towering humanoid robot fit for parody alongside Godzilla in a tag-team environment. The colorful Jet Jaguar is inconceivably stupid, included for its ability to change the rules on the fly and make it easier on the script-writing team in a rushed time frame. After all, Ultraman-obsessed kids would flock to the theater anyway.
For all of the mayhem – which is mostly stock footage from previous monster epics - Megalon still thinks it can carry a message against nuclear weapons, even though that train had left the station. Seatopia is a picky nation when the surface dwellers take their pot shots, and despite being at war with an entire civilization, the outcome comes down to two brothers plus their... nephew? It's never explained.