Adventure Time is a victim of the Seinfeld Effect
In discussions on TV shows that started out boring and turned out incredible someone always brings up Adventure Time as an example and it drives me insane.
The Seinfeld Effect refers to people who were raised in a post-Seinfeld world thinking "Seinfeld is unfunny" because they don't have the cultural context to understand how influential Seinfeld was. I think the same thing happened to Adventure Time.
Adventure Time was not in any way a normal, boring, standard cartoon for its time. The pilot went viral on YouTube because it was unique and unlike anything seen before. Despite going viral Nickelodeon rejected it, perceiving it as too weird and a huge risk. Weird cartoons with serious themes were not seen as profitable.
Adventure Time was fun, unique and interesting right from the first episode. It's unlike any other fantasy series that happened before, and the creative chaotic atmosphere builds the foundation from which the later lore and character development could happen. The later series world building was possible because the world was already interesting due to years of creative freedom and weird ideas.
The creators who worked on Adventure Time went on to create a lot of other influential cartoons like Steven Universe, Over the Garden Wall, Clarence, and Regular Show. Other unaffiliated cartoons and comics also were strongly influenced by the style of Adventure Time. This has become normal now but it was not normal when it first premiered.
On the other hand Chainsaw Man was influenced by Adventure Time and look at how that turned out.
I think the lore and relationships have taken up too much focus in the Adventure Time franchise, particularly in spin off series like Fionna and Cake, and while I don't want to drop the drama entirely I think the franchise would be better off if it could recapture some of the insanity of the first season instead of 12 hours of Simon being depressed. So that's another reason it frustrates me to see people treating the early seasons like they are unwatchable.