Is this something that only I perceive, or is it real?
It’s not just you. See 501715 – Text rendering in Plasma UI components using Text.Rendering does not respect system's hinting and sub-pixel anti-aliasing settings and Set default text rendering to "native" (and allow env override) (!5689) · Merge requests · Plasma / Plasma Workspace · GitLab.
I just noticed this myself, but I don’t even use fractional scaling nowadays, I made a reddit post: https://www.reddit.com/r/kde/comments/1mpqdeh/fonts_blurry_on_krunner_and_overview/