Evil Is Hammy: In "King for a Day", Prince Gasket brainwashed Tommy into thinking he was their king and that the other Power Rangers were his enemies. He then proceed to act just as hammy as he did when he was the evil Green Ranger.
Evil Laugh: Got it back when he was brainwashed in "King for a Day".
Real Life Writes the Plot: David Yost reportedly didn't like the slurs against his homosexuality on the set and walked off before the writers could make up a story to write him out.
Rapid Aging: Apparently the explosion messed with his molcule more then he thought as they aged him into an old man. Forcing him to have to go with the Aquaians homeworld to restore his youth. He winds up staying there when he falls in love with one of the female scientists and that sadly is the last we see of him...
Chuck Cunningham Syndrome: He was last seen in "Rangers of Two Worlds, Part 2" unsuccessfully facing the monsters controlling the Megazords. Unlike with Ninjor, the show didn't provide even the slimmest explanation of where he went afterwards.
Ernie | (Actor: Richard Genelle)
Ascended Extra: After three years of being a recurring character, Ernie was promoted to main cast for Zeo.
Bulk & Skull | (Actors: Paul Schrier and Jason Narvy)
Badass Normal: In "King for a Day, Part 2," they fended off Cogs, the mooks that Rangers preferred to fight while morphed.
Ascended Extra: As with Ernie, Stone was promoted to main cast after being a recurring character.
Bad Boss —> Benevolent Boss: Played both ways. He's prone to yelling and punishment, but of course, he's the boss of Bulk and Skull. Given their antics, anyone else would've fired them immediately, but Stone obviously has a soft spot for them.
Badass Normal: In "Graduation Blues," he stumbled upon Goldar and Rito - thinking an invasion was a foot. He chased them all over the park to try to arrest them. Granted, the two were amnesic and not up to no good, but Stone didn't know that.
Motive Decay: When the Machine Empire first attacked Earth, it's because "our solar system is the final link in a chain of galaxies already conquered" by them. Conquering Earth would effectively give them control of the universe (or at least enough of it to prevent any opposition). The Zeo Rangers were initially viewed as an "interesting diversion" to make the conquest more entertaining. By the end of the season, though, Mondo is more interested in defeating the Rangers. Possibly justified in that the Rangers repeatedly defeated him (including briefly destroying him), making it personal.
Orcus on His Throne: Like any other Ranger Big Bad, he directs monsters against the Rangers. The trope is subverted, though, when he does fight the Rangers in "Mondo's Last Stand" and "Good as Gold." They wind up having no more trouble beating him than the average monster.
Big Bad Wannabe: He sought to become leader of the Machine Empire in King Mondo's absence, but failed to achieve any serious victories that would allow that ascension. Plus, he beat a hasty retreat after Mondo came back.
First Name Basis: Every major villain will usually know the Rangers' names and tend to address them as such, but Gasket took it a bit farther. He refers to the Red Zeo Ranger not as "Tommy," but his actual first name ("Thomas").
Long Bus Trip: When Mondo finally cornered him, Gasket fled with Archerina and never returned.
Real Life Writes the Plot: The costumes for both him and Archerina practically disintegrated. Some parts were salvaged for later use, but the complete suits were irreparable.
Disproportionate Retribution: She developed a grudge against Kat (the Pink Ranger) very quickly (even though the two had never previously met), simply because she detested the fact that there was another female warrior around who liked bows and the color pink. (That's right. This was a rare case where the villain was actually upset with the realization that she and the heroine might be Not So Different.)