Adventure Time is a show that probably better explores complex ideas and gets into adult themes and concepts without alienating young viewers. Steven Universe is made with higher production value, so it looks, sounds, and feels more polished as a show. Which show is better really depends on your tastes.
People who enjoy Adventure Time watch it because it has clever humor, dark undertones, and finds approachable ways to explore heavy topics. People who dislike it often are turned off by humor aimed at kids and an odd art style. Whether or not it’s worth watching really depends on your specific tastes.
Adventure Time is definitively a cartoon that was made for a young audience. Despite that, it has many elements that cater to adult viewers, and that is why many adults have enjoyed the show for more than two decades now. It is for adults, but it’s even more for kids, and the kids’ elements do turn away some adults.