While oddly none of the main characters seem to have a catch phrase in Cwen's Quest it almost seems to be a requirement for the minor characters causing fans of the series to repeat them whenever a minor character show up.
One of Fred The Monkey's favorite tropes. Fred's, "Eee? Pah!", Sceb's "What the biscuit?" and, of course, Clydex's "I'm a platypus."
Othar Tryggvassen, Gentleman Adventurer! In addition to bellowing his name, Othar generally yells "Foul!" when a Spark defeats him in a manner that doesn't fit the rules of hero/villain combat that exist in his head.
Also, when somewhat-less-mad-than-regular scientists Agatha and Gilgamesh are thrust into the stereotype of their role, they tend towards the line "...and I can live with that!". (Things they've been able to live with thus far have included death rays, ranting at underlings like a cheap actor, and controlling an entire city and its army via a mix of mind control, symbiosis, and shared insanity.)
In the webtoon Geo Weasel the character Mitri once claimed to be "Pretty much everywhere and nowhere, like a ghost or something." In later episodes, his brother would pressure him into saying it again, and then laugh like an idiot and proclaim it to be the funniest line in the world. This was probably a parody or a Take That aimed at several of the toon's fans who thought the line was so funny that they pressured the creator into adding it again.
Jimbob also had the Catch Phrase "Fix my saxophone", but it was parodied to the point that it became very irritating to everyone in the cartoon whenever he asked. Weas questioned whether the line was actually supposed to be amusing at all just by repeating it over and over and if it was leading up to anything (to which Jimbob replied cryptically "episode 15"). Later uses of the phrase either had someone stop Jimbob before he could say the whole thing or even ask or he would receive punishment for it (such as Weas shooting him dead in the first episode and stabbing him in the face in the long-awaited episode 15).
In Heavenly Nostrils, Marigold the Unicorn cheers "UNICORN!" whenever she does something awesome... which is fairly frequently.
Bert: "My world is a crotch!" Later, when he shows up again as a ghost (for a short time), it is confirmed to Torg that it's him when he says, "My afterlife is a crotch!"
The Unspeakable Vault (of Doom): The shoggies reaction to pretty much anything is "Oooh! Sooo coool!", but admittedly they are as dumb as you expect brainless pink goo to be.
Spoofed/subverted early on in Wigu, when the title character learns that he must be careful not to utter the catchphrase ("Spring into action!") of his cartoon hero Topato Potato, lest he be hunted down by the Catchphrase Killer.
Jason from The Word Weary answers "Who Cares?" off-handedly whenever someone asks him about another character's emotions.