Regarding mature kids cartoons, I really hate it when people (both the audience and writers) excuse bad/poor writing as being the median for kids and should always be expected, say a kids cartoon isn’t actually for a kids because it has mature themes or kids shouldn’t watch it, or excuse an opinion because that person is not a kid and not the target audience so therefore your opinion doesn’t count, and this catch-all is the first ‘counter’ towards any criticism.
Yes, it’s a kids show. It’s also an adult show. The phrase, ‘mature kids cartoons’, is not an oxymoron.
Steven Universe is a kids show. Both Avatar series were kid shows. Gravity Falls was a kids show. All three Star Wars cartoons from the last roughly-fifteen years are kid shows. All DCAU series were kid shows. I could go on.
Kids are not goldfish. If a mature subject is brought up and explored, then they have something to think about and be taught about.
For example, if you show them something like a character suffering from a poor choice, they’ll know that choices matter and can have serious consequences if you choose poorly.
And making/saying that poor writing choices as being dumbed down for them is why it happened, you, no matter if you are a critic or writer, are not respecting how smart kids really are and deserve a better story.