If you go back to the likes of Duck Tales, Dexter's Lab, and Animaniacs in the late 1980s and the 1990s, then yeah, they were better.
If you go back LONGER, to the likes of Transformers, G.I. Joe, and He-Man, then you can make a pretty good case that today's cartoons, while not great, are generally better. There's a lot of people that love these, but where you can go back to Animaniacs and Duck Tales and appreciate them all the MORE for the life lessons and adult ideas that you didn't necessarily understand when you were a kid, you can't say there's the same level of content in this bunch of cartoons, which amounted to mindless and repetitious action.
If you go back EVEN LONGER, to the original Loony Tunes and Disney shorts from the 1950s and earlier, then you can make the case that older cartoons were better.
I'd say it's a cyclical thing and not a linear path of degradation. That and, well, every age tends to be a bit of a mixed bag.