Base Breaker: Gruffi and Grammi qualify, given how they're hard on the younger Gummis.
In addition, any human in Dunwyn who isn't Cavin is this, given the high standards of knighthood, as well as their authority. Calla on the other hand is also a Well, Excuse Me, Princess! in addition to this trope.
The Knights of Gummadoon, specifically Sir Plucki, Sir Gumlittle and Sir Blastus, only show up in one episode and yet appear in tons of fanfiction and fan art! Quite a bit of fanfiction finds ways to bring them back into the main cast.
Chummi Gummi, who shows up once in "Up, Up and Away" is a fan favorite.
Germans Love David Hasselhoff: Czechs love, nay, absolutely adore Disney's Adventures of the Gummi Bears, and they have taken their dedication to amazing levels. The show is popular everywhere, but consider this: The translation was of extraordinary quality, full of intelligent and hilarious word play and pun names. The cast of voice actors was made of awesome as some of them were really famous Czech star actors. The catchy theme song was recorded by a well-known singer. Fans, now grown up people, keep arguing whether Cubbi's Czech name is Bida (it's a variation of a common nickname for bears) or Pida (it's an affectionate pet name for cute and little things). All the Gummi Bears' Czech names start with a "B", which resolves the conflict but Fandom is a Serious Business and True Fans never give up their fight.
Ogre decay. Early on in the series, with one exception, the ogres were regularly portrayed as capable of feats of extraordinary strength. Later on though, they needed Gummiberry juice to do anything beyond the limits of a human. And the weird side effects that the juice initially had on ogres was apparently forgotten.
Lady Bane was malevolent and competent in her initial appearance, but later episodes featuring her have her fail just as badly as Igthorn due to Flanderization of her vanity issues.