I always associate the term "filler episode" with anime, since there it means anime-original content added into the show to keep it from catching up with the ongoing manga. But if you don't think watching Goku and Piccolo learn how to drive in a DBZ filler episode isn't the best shit ever, then we're gonna have some problems here.
I think that when people refer to an episode as "filler", they mean the episode is "unimportant" or "skippable." The problem is that what's unimporant or skippable can be subjective.
Garnet's Universe has some nice animation at times, but it's not terribly important in the grand scheme of things. However, the real purpose of the episode was some very subtle development of Garnet and Steven's relationship. Steven describes Garnet as wanting to tell Steven how much she loves him. Garnet actually realizes after his story that he wishes she was more open with her emotions with him. After the episode, you do see Garnet open up a bit more to Steven. It's very subtle though, to the point that I would argue it's not intentional and that Garnet's Universe and subsequent episode just didn't do a good job of establishing this change in Garnet. Say Uncle is filler, but screw continuity. Sometimes you just want a show that has gotten more serious to let loose for an episode with bat-shit meta humor and Pearl's personality magnified by 1000% percent. I can tell that the crews had fun making the episode. So even though Say Uncle is literally not canon, I'd be more likely to tell new fans to skip Garnet's Universe than Say Uncle.
Future Boy Zoltron might set up stuff for future episodes with Smiley and Frowny, but it feels so utterly, utterly pointless at the moment. It's my second least favorite episode, beaten only by Onion Gang. As much as I loathe Onion Gang with a burning passion, it accomplishes its purpose of establishing that Onion considers Steven a true and deep friend. The wooded area where the gang meets also comes into play in Are You My Dad?, since that's where Aquamarine and Topaz capture Connie. Onion being the first on-screen abductee of Topaz and Aquamarine has more impact thanks to Onion Gang's focus on Steven and Onion's relationship. Onion Gang is my most hated episode, but I've had to eat crow and admit that it set up stuff for the future (including possibly foreshadowing some deep lore or main plot developments if you believe some fan theories).
Rocknaldo, as much as it's derided by fans, is one of my favorite Ronaldo episodes. He's insufferable in this episode, but that's the point. He's trying to be a better person by recognizing that he's been wrong about the gems, but he takes it in the totally wrong direction and makes it all about himself. It also has the Gems playing a role in the episode, leading to some really funny lines like Amethyst's "You truthed him so hard that he died!" It's appropriate that the Gems literally just dump him like trash in front of the fry shop in the middle of the night. Anyway, the episode isn't filler since Ronaldo has developed as a character by the end of it into a more functional and well-meaning supporter of the Crystal Gems. Hate him or loathe him, Ronaldo is one of the more prominent townies and he'll undoubtedly play some sort of role in more "important" episodes to come. Even Uncle Andy got to be a part of the Zoo arc by flying Steven and Greg to South Korea.
I think the major problem with some fans that they're expecting one kind of show all the time and getting angry or disappointed when their expectations aren't met. This can be with specific, asinine stuff like getting angry when an expected event doesn't happen or when a character is acting in a way they don't like, despite the fact that the character is supposed to be in a grey area or setting up character development. They react impatiently without letting the story play out in full and without looking at things from a different perspective. People were mad that Steven cut off Pearl's revelation in Gemcation to cry about Connie hating him. But Steven's a young teen going through his first psuedo-break-up/fight. Of course he's going to think his problems with Connie are the worst and only thing in the world right now.
