This is from documentation:
Clarity includes several SASS variables, collections, mixing, and functions for working with typography. These are described below:
$clr-font
This SASS variable points to our default text font, Metropolis.
I don't know how to change it.
All of the scss variables are customizable but you need to overwrite them with your preferred values. This can be done with a custom theme. The docs have details on how to set this up for an app built with the cli tooling: https://clarity.design/documentation/themes