My experience, is that the ones made in the 1970s and 1980s had crap quality.
I watched this movie, and think it very much deserved the Oscar, but the character rendering was a bit “scruffy.” The environment rendering was great, and it looks like they optimized for movement, in the characters, which was a good choice. Once I spent some time, watching, the rough rendering didn’t matter.
I had a similar experience, watching Avatar. At first, it seemed like a cartoon, but I quickly became immersed, and the fact it was rendered, didn’t matter.
I read, somewhere, that the movie is being re-rendered. I think they may have the money for that, now.
Crap quality is typical of cheap TV productions, e.g. Hanna-Barbera and some anime in the seventies and eighties.
Many modern cartoons are 3D-rendered, and I feel a bit "uncanny-valley" about them. That may be, because I was raised on the classics.
Because a lot of it turned into a marketing machine thanks to GI Joe. Cheap cartoons enabled kid oriented commercial slots to sell ad time for junk food and toys. The 80's were notorious for throwing all sorts of action figure selling ideas at the wall. Every 80's kid had some cartoon merchandise toy crap.
Animation tools are just part of the story-telling tools, and just because something is visually beautiful in stills (or even animated) doesn't mean that the story is well told, or the tools well used.
And often 'bad graphics' or whatever you want to call it can actually help with the story, just like low-def TV, because it covers up things that are unimportant without drawing attention to it.