Currently, skin Minerva Neue (default skin for mobile) assigns
-apple-system, 'BlinkMacSystemFont', 'Segoe UI', 'Roboto', 'Lato', 'Helvetica', 'Arial', sans-serif;
as fonts for paragraphs. (Source code.)
This is suboptimal because it does not allow user to display the content in their preferred sans-serif fonts in settings.
In contrast, the desktop version of most Wikimedia sites (using skin Vector) only has "sans-serif" configured, so it will use whatever sans-serif font users choose in their browser or system settings.
One particular example I want to mention is reading mobile Wikipedia on Windows. Because of how the rule is set, it will use "Segoe UI" (instead of something like Arial, as most of browsers default to), which is not very optimal for article reading.