The answer to "should I open a GH issue?" will pretty much always be "yes"
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Very few things are actually platform-specific.
In this specific case, KOreader's "system" fonts setting is hidden (and force-enabled) on Kobo (and most other !Desktop platforms). With a caveat: "system" fonts are actually unusable on Kobo (they're obfuscated/encrypted). What is enabled by default are "user" fonts (i.e., what's in /mnt/onboard/fonts), same as Nickel.
That said, we ship Droid Sans Mono, so the "monospace" alias should point to that if nothing else, IIRC.