Formulated more rigorously, cross-platform GUIs and outsider, non-Mac-first GUIs ported to Mac OS look (and feel) bad on Mac. The opposite is virtually never true though; there aren't really high standards for beauty or consistency on the other platforms. Windows, for example, in this decade is a mishmash of different toolkits (even from Microsoft). Desktop GNU/Linux people comprise a faction consisting of people that either doesn't care about GUI beauty or have standards that are about on par with Windows folks—and are generally so grateful just have an app that ships* their platform, that they won't reject outright any Mac-first app (and that would be true even if it painted itself as a pixel-for-pixel match of the Mac OS version).

* and runs; I still run into "cross-platform" apps that are Electron builds packaged as AppImages that still terminate at launch, even if you try to run them on something as unremarkable as Ubuntu