It is quite plausible, that the quality problems of Catalina and iOS13 at the launch were a consequence of shifting the development team around to also support the development of MacOS on ARM.
On the surface, there isn't so much new in Catalina oder iOS 13, that you would think that things break so badly, but they did.
Starting a new branch of the development always is a disruption. Developers would be reassigned to new groups, probably the best ones even, new developers would join the existing groups. This process probably has started like 2-3 years ago and intensified like 1-2 years ago, especially after they had the first silicon to play with - and I assume the CPU will be sufficiently different from an iPhone so that you want to at least optimize your code for it, there might also be entirely new features to support.
mac os at large has a quality problem. I have gotten more kernel panics on my personal and work laptops in the last 6 months than I have on my windows machines in the past 5 years.
I think the difference is, that Apple threw everything they could to fix iOS13, as their survival depends on it, so it did stabilize, while Catalina still hasn't recovered.
1. mac os has been buggy for quite a few releases already
2. outside of drivers and apps, ios and macos are basically the same kernel and userspace/libraries, so there isnt much to port