That's stupid.

Just about every single 20 something was watching Adventure Time when it was airing and that is also a kid's show. If something is well written, and appeals to a broad audience, it is well written and appeals to a broad audience, regardless of intended audience.

As for obsession, that depends what you call obsession but as a neurodivergent person (didn't know I was for a huge chunk of my life but that's irrelevant) some of my interests have always been intense or as you might say, obsessive, and idk about anyone else who may have interests considered obsessive, but personally I'm done apologising for how my brain works and I will never tone down my interests or mask who I am again. Otoh, if someone is "obsessed" who is neurotypical they are then probably just a passionate person or really like a particular show which is also a fantastic thing anyway.

There's nothing wrong with really liking your interests. And being a kid's show shouldn't change that. If something is good, it's good and it would be childish to deny you like something as much as you do just because it's aimed at kids. Doing so gives off "I'm so mature and I prove it by disliking things even if they are good" vibes (which typically indicates the person is actually not mature).