All widgets outside the editor pane follow the GTK theme, for better or worse.
But you can set font-family for the statusbar in geany.css, there is even a commented out statusbar section at the end of that file[^1], and an example near the top of setting `font-family: monospace`.
[^1]: at least it is in the git version, not sure if its released, but you can read it on github if not.