This is a follow-up of https://phabricator.wikimedia.org/T67532
Of course this is a recurrent bug: native MathML rendering is very poor without appropriate font with a MATH table. Even with an OpenType MATH font, using a font other than "Latin Modern Math" gives result different from the default "Computer Modern" style used by the SVG and PNG output. Users who are not aware of that get inconsistent results. We can always improve the doc to tell them to install fonts, but that won't stop people from complaining.
Currently, the only OpenType MATH font installed by default is Cambria Math on Windows, but it sometimes give bad rendering results in Gecko (especially for square root IIRC). Also, I opened various enhancement requests to get math fonts installed by default on other systems but that does not seem likely to happen anytime soon...
So the alternative, on the MediaWiki side, is to always have a fallback with Latin Modern Math Web fonts. However, using big Web fonts on MediaWiki server has been controversial in the past, so I'm not sure we can do that. But I'm opening this bug as a reference for future changes...
Current status: As I read the code, the default font-family on the math is
font-family: Cambria, Cambria Math, Latin Modern Math, STIX Math, LatinModernMathWOFF, serif
and LatinModernMathWOFF is only loaded on desktop. So the Web fonts are likely to be used only on Mac/Linux. Extending that to mobile & Windows is likely to significantly increase server requests.