Saving this as a post here for my future reference.

The key to getting the “default” fonts to apply to plain-text (and all mails with the allow messages to use other fonts setting off) is also changing the fonts for Other Writing Systems.

Why is it like this? Who knows!

This answer brought to you by a semi-not-recent reddit thread.

[–]Yukness   2 points 3 years ago

The display fonts are set in the Advanced section of Language & Appearance in Options. Did you set the fonts with Fonts for-Other Writing Systems? If not, that might explain why Unicode plain text in received mail doesn’t look like what you specified.

Shoutout to /u/Yukness for this one!