What was part of the problem was patent-encumbered rendering engines for a long time.
Fonts weren't what held the desktop back, what held it back was that it just didn't have the resources that commercial enterprises were and are able to pour into their own efforts.
With commercial, you have a manager in the boardroom that says here are the whiteboards of how it should look, now go make it and have it done by next week so marketing can tell everyone this is what they wanted. With open, you have a bunch of introverts who have to agree to talk to each other and not hate the other guy's idea too much.
Nevertheless, the default fonts can be deleted and replaced with nice fonts, which can make any Linux look better than Windows or Mac OS.
I have been using for the last 20 years only Linux on all my desktops and laptops, but since the very beginning I have never used the default Linux fonts, but I have always replaced them immediately with beautiful fonts.
When I have started using Linux, it was much more difficult than today to find good free fonts, so I have bought many good typefaces from companies like Linotype, which no longer exists, because as mentioned in this article it has been bought by Monotype, or from Adobe, which appears to be the last big commercial vendor of typefaces which has not been bought yet by Monotype.
Nowadays, it is much easier to find good free fonts. Especially for programming and CLI windows there are a lot of very good free fonts from which to choose.
For proportional typefaces, it can be a little more difficult to find good free fonts, though there is always the solution to grab some fonts from Windows or Mac OS. I have stopped using Mac OS more than a decade ago, but I have still kept from it a Japanese typeface that I have liked and then I have continued to use on Linux, while from my last Windows I have kept Palatino Linotype for polytonic Greek.
So if some people use shitty fonts on Linux, that is their fault, not of Linux, because it cannot be expected for a free product to include good licensed typefaces, like those whose price is included in that of a Windows license or Mac OS license.
It's definitely rather idiosyncratic, mind you.
So I don't personally use it on Linux. I use Google's Roboto, which is close enough to Apple's later San Francisco (which shares some common heritage and some common modern touches) that I don't go insane when switching between the two!
Roboto is again a considered bit of work by a highly competent designer.
I don't have problems with either. If I did, I could use the Fira suite, which is lovely.
In a world where Arial Illegible is the standard, I don't think we can complain too much.
FOSS doesn't as a whole pay much attention to visual aspects, one reason so much of the software just doesn't look nice. E.g., GIMP.
linux distros were some of the first to use proper LCD hinting and anti-aliasing on the fonts, so they always looked buttery smooth and polished when compared to the Microsoft offering at the time.
Funny how things shift around.
Although I think part of the problem is just how much of a nightmare Fontconfig is. I dealt with fonts on Windows and macOS, and there were a ton of hoops you have to jump through if you want tight control over how you display text on-screen. But Linux was in a whole other realm altogether—you used Fontconfig to select your font, and Fontconfig is truly, truly awful.
Yeah, I know what its capabilities are. But the best software which uses fonts on Linux tends to do so bypassing Fontconfig.
It was slow, slow, slow getting good font rendering on Linux.
It happens that many of the free fonts are crap, and part of the reason Linux on the desktop never took hold was lack of good bundled fonts. As it turns out, good fonts cost money.