It's should be simple to change it to cosmic::font::FONT_MONO_REGULAR (which is Fira Mono), I don't think it'll look out of place compared to the default font (Fira Sans). Although when I tried it with .font(cosmic::font::FONT_MONO_REGULAR), it doesn't seem to find it and fallbacks to a different font, but I'm also getting no warnings or errors at all so I'm not sure:

Also apparently I found a bug where the screenshot continues to the wrong monitor, I have two monitors and secondary one is to the right of the primary, but after moving the mouse too much left in the selection it captured the right area of the right screen (if this is intended to wrap around like that, there's currently no visual indication that it's happening).

it might not be a good idea to hardcode a font like that if they want to make fonts configurable

Perhaps an option to use monospace font in the Time applet settings UI that is on the roadmap.

I don't think an option is necessary. Why not use a mono font by default?