Mendix sass should be updated to not use px, but use rem instead for font sizes.
Px is an absolute unit, so it does not scale depending on the device size or a users text size preferences. Where as Rem will scale to a users viewpoint.
The most notable reason for this change is for accessibility of users who have different browser font size.
In most modern sites (which use rem), if a user changes their text size to large/very large, the font size on the webpage will respect this and update.
Out of the box in mendix it will not change, unless a developer updates the sass to use rem.
According to stackoverflow, this was true up until around 2010.
Modern browsers understand screensize and resolution a lot beter then they did back in the day (12px font on a retina display is 24px high, cause retina magic).
There is also nothing stopping you from changing your variables to REM, most atlas variables are derived from the variables managed in Custom-Variables.scss.
I hope this helps, and also a great read from stackoverflow regarding this: