Joplin allows you to override the CSS directly, so you can choose fallbacks. Examples of various things people have done can be found here: https://discourse.joplinapp.org/t/share-your-css
FYI, currently the WYSIWYG editor cannot be customized, but maybe by using the trick above you can keep Markdown in a monospace font.
For changing the font size, you can zoom the window. This setting is found in the View menu or by hitting Ctrl+ / Ctrl-. Depending on your interest, see my note about CSS above.