No, it isn't. Lots of non-trivial OSS desktop applications are clearly made by people with no interest in aligning with expected desktop GUI behavior. From Gimp with dozens of windows to LibreOffice which is slow and has bad font rendering. And those are the 'poster apps' for FOSS desktops, lots of apps are worse.
Gimp's single window mode was made the default years ago now, so that's not a great example anymore - there's scientific software that uses that paradigm that might work better, but most of that isn't OSS. Also, Libreoffice being slow and having bad font rendering seems pretty inline with Word nowadays...
Amusingly enough, the only OSS desktop applications with good UIs got them by shamelessly stealing from commercial software that had actual paid engineers and UI/UX researchers.
The best way to draw a circle in gimp is still the awkward select -> foreground fill workflow. At this point this example is beating a dead horse, but the horse shall continue to be beaten until a proper ellipse tool is added.
Depends on your system. A few years ago I ran it on a MacBook where scrolling on an empty page took ages. Seems nobody tried it out on a Mac before releasing the port since it was totally unusable. Hopefully it's fixed now, but I wouldn't recommend a piece of software I don't trust to anyone.
No, it isn't. Lots of non-trivial OSS desktop applications are clearly made by people with no interest in aligning with expected desktop GUI behavior. From Gimp with dozens of windows to LibreOffice which is slow and has bad font rendering. And those are the 'poster apps' for FOSS desktops, lots of apps are worse.