The reason modern cartoons suck is because the type of competition has changed. The 90's were all trying to get people to stick with their network to build a long-term fan that could consume merchandise and watch ads. All you needed was good ratings and they had to compete with several other networks at the time.
Now views are split across several different platforms and all these networks care more about is a short-term binge-watching clicks after they pump and dump and then greenlight another show to take its place. These networks and streaming services would rather have 50 shows with a season each that people will watch and forget than 10 shows with 20 episodes each. On top of these, the amount of parent companies has shrunk and they're all doing streaming deals with other services.
Nickelodeon is no longer competing with CN, three channels owned by Disney, MTV, WB, FOX Kids, and whatever the fuck else was airing at the time. Now they're just competing with Disney and CN. So the quality of show goes down since they don't have to try as hard to make something unique, just make something that works to get viewers watching again.
Doesn't help that ratings for kids shows have been dropping every year.