I am not a designer, just a dev with a bit of a visual inclination, but I think what makes KDE’s typography feel “off” is missing steps in visual hierarchy due to how the different sizes relate to each other in screen (e.g. header vs body text), as well as font weights being a bit too homogenous. So its differentiation is too great in one way and not enough in another, which feels awkward. The font sizes being modifiable isn’t the issue.
There's a community effort underway to develop a next-generation appearance based on a design system (and a new theme engine) that also includes a properly designed set of font weights.
I agree this will be a big improvement. I love type design, and our typography in KDE being relatively scattershot has always been one of my pain points as well.
