The beauty of Steven Universe lies as much in its sparkling animation and earworm-worthy music as it does in the insight and complexity it gives to its characters. None of them are one-dimensional in that they are not limited to a single emotion (maybe except Jasper) — in fact, the story takes all of them on a journey of self-discovery and personal changes.
One of the many aspects of these fan-favorite characters is the unique brand of humor each of them possesses. A lot of the narrative gravity in Steven Universe revolves around the various perspectives displayed, as well as depending on the hilarious ways in which different characters react to the events taking place around them.
Lion
Lion doesn't speak a lot, and his devotion to Steven is obvious to everyone around (as seen from his protective stances whenever his friend is in danger). However, for the most part, he spends his time acting like a domestic cat, which is just hilarious to watch, considering the size of his body, and, more importantly, jaws.
Lion runs after butterflies and other shiny objects, plays with his tail, and does a whole range of cute things in the background while the show goes on.
Lars
Lars starts off crude and malicious, but Steven doesn't seem to mind his harsh words at all. Besides, it's later explained that he only pretends to be mean because he didn't want others to notice his insecurities.
As such, Lars sometimes finds himself in embarrassing situations that are often quite funny. It's especially great to see him act all cocky and overconfident about something, only to be brought down a couple of pegs (in a completely gentle, good-natured way).
Pearl
Pearl is not necessarily a funny character in a direct sense, but the various aspects of her personality — unnecessary sophistry and extremely specific demands combine with her easily bewildered sensibilities when things don't go according to the vision in her head, creating some choice moments of comedy.
Generally, however, Pearl is far too serious to play around like the others, but her insistence on protocol is occasionally a bit too much. Still, this also forms part of the whimsy she exhibits.
Greg
Greg is a compassionate man who loves nothing more than his son, but his borderline childishness is played for laughs. His favorite motto, "If every porkchop were perfect, we wouldn't have hot dogs," is meaningless, but the way it's phrased is rather hilarious from an absurdist perspective.
Greg is the prototypical goofy dad; the best thing about him is that he rarely seems to care what others think about him or his character. The only thing that matters to him is maintaining a close relationship with Steven.
Smoky Quartz
The fusion of Amethyst and Steven is the adorable Smoky Quartz, whose playful nature is derived from a blend of their original components. The interesting thing is that they don't find it odd to make humorous observations during combat, and their cheerfulness is so overwhelming that it helps bring back some of Amethyst's self-esteem.
Smoky Quartz's jokes are, at times, made at their own expense, like referring to themselves as "one big super-wrong, good-for-nothing dud with a yo-yo," which is not as depressing as it sounds, really.
Peridot
Although Peridot's introduction cements her as a scheming villain, this image is broken down over the course of her time on earth, living with the Crystal Gems. A large fraction of her comedy, to be honest, is obtained from her manic tendencies towards overreaction and hyperbole.
Peridot, being a highly patriotic citizen of Homeworld, refuses to learn human culture, which results in her completely misreading references, jokes, idioms, and so on. Further, her immense knowledge about engineering is diluted by the fact of her belief that earth consists of a new variety of species known as "Steven."
Ronaldo
Ronaldo's aim in life is to "keep Beach City weird," a blog that is by itself one of the most amusing things in Steven Universe. He plays the role of a sweet but naive teenager, who loves everything about conspiracy theories.
Unfortunately for him, his deductions are regularly off base, like when he suspects that Steven might be pregnant, or that he might be some kind of serpent-human hybrid. There is no logical grounding to Ronaldo's theories, but rather than seeming like a total crackpot, he comes off as this kooky comic relief character.
Steven Universe
Steven is quite funny himself; one of the first scenarios in which this trait of his emerges is when he begs, pleads, cajoles Pearl and Amethyst to become Opal, even making a brilliantly ludicrous song about wanting "to see a giant woman."
In fact, Steven constantly lapses into cute lyrics to express his desires, and any immaturity he demonstrates is usually resolved with him learning something important, albeit in a very entertaining manner.
Padparadscha
The innocent Padparadscha is an off-color Sapphire, whose powers are the exact opposite compared to those of her kind. In short, she predicts the past rather than the future, which is mostly useless in a battle situation. The funny thing about her is that she is constantly stunned by her own forecasts, implying that she might actually experience the passage of time in a different way than everyone else.
Interestingly, however, Padparadscha has made a few predictions that have been helpful, for example, when she claimed that Lars' accidental button-mashing is the reason that the core of the Sun Incinerator isn't working, which nobody else had realized.
Amethyst
The most hilarious character in the series is arguably Amethyst — whose complete lack of interest in the concept of seriousness is both amusing to behold as well as an occasional hindrance during Crystal Gem quests. Her antics always make Steven laugh, and even brings Garnet into snicker-mode, but Pearl, however, is often vexed by her inability to keep things in perspective.
Amethyst's Purple Puma wrestling alter-ego is uproariously absurd, not to mention how much she enjoys playing tricks on her friends (in her defense, they are merely ways for her to express affection).