Its much more difficult to use custom fonts on mobile. Not all fonts are available on Google Fonts, and even if they are, using @import to add them wouldn’t work if the device is offline when the Obsidian mobile app starts.
Obsidian should have a setting page where users can import font files using the OS’ file picker. Ideally it supports OTF and TFF, and it should support WOFF since Chrome supports that too.
Once a font is picked, users can select it in the Appearance menu, and be able to use it in CSS.
Current workaround (optional)
Its possible to install them on iOS with 3rd party App Store apps, but it varies a lot on Android devices, with different ways of doing so, and on some devices, its not possible at all.
That is a workaround, but its a lot of work to navigate through a long base64 string in a CSS file, and that does involve knowing CSS. If Obsidian wants to be more powerful to non tech-savvy users who don’t know or care to touch CSS, then this FR should be implemented.