@Chris Network Admin, Thanks for posting in Q&A. For outlook on iOS, I don't find the app configuration policy to set the default font.
You can feedback to Microsoft 365 to see if we can have this setting in the future.
On Windows, based on my researching, I find threads describe using PowerShell script to change the font. Maybe you can try to see if it can help.
https://www.joeyverlinden.com/default-fonts-and-styles-for-outlook-via-intune/
Note: Non-Microsoft link, just for the reference.
Hope the above information can help.
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