I have looked in mpv.conf and in input.conf and don't see any setting for subtitle font. I see size but not font name.
However I see that it must be a configurable value because I have mpv running on two different machines and see they are different from each other.
The reason you see different fonts is that video files can provide their own fonts, but it is also possible to set your own font. It is documented here: https://mpv.io/manual/master/#options-sub-font
--sub-font=<name>
Specify font to use for subtitles that do not themselves specify a particular font. The default is sans-serif.
Examples
--sub-font='Bitstream Vera Sans'
--sub-font='Comic Sans MS'
Note
