And I think this issue can be traced right back to the schizophrenic nature of TAS. The ideas and stories were often adult level, but the overall execution was touched by a mentality that saw the show as not much better than kiddie fare. It pisses me off knowing that a little extra thought and effort could have made a significant difference yet that little extra wasn't thought worth it.
I don't know where you're getting this assumption from. It's not like there were really any animated US television shows at the time that were made specifically for an adult audience, so you've got no standard for comparison that would make this conclusion remotely valid or fair. Indeed, I would submit that TAS was the most adult-oriented animated production you would've found on American television in 1973-4.
And I'm not sure what it is that you consider the result of inadequate thought or care. If you're referring to the quality of the animation or the performances, I've already told you that the network imposed an insanely tight production schedule on Filmation, so they were forced to rush it. And even so, the quality of the artwork and design was better than anything Filmation's contemporary/rival studios were doing.
And if you're saying that you consider the fanciful elements of the stories to be childish, let me remind you that TOS gave us Alice and the White Rabbit, a superpowered alien brat with a Napoleon fetish, a haunted castle with a giant black cat, a Greek god, a gangster planet, a Nazi planet, multicolored disembodied brains gambling on fights, and Abraham Lincoln floating in space. Yeah, there are a few TAS episodes that get somewhat more fanciful than TOS did, but not that many.
Hell, a lot of people think the film Fantastic Voyage was produced and/or directed by Irwin Allen!
I can understand that mistake. The film was from 20th Century Fox and used basically the same in-house effects team that later worked on Allen's shows, so a number of props and set pieces from FV were recycled in Allen's shows.