I have a have 27" UHD monitor (3840×2160, DPI set to 192 manually). When I set the scaling value (System Settings > Hardware > Display and Monitor > Scale Display) to "2", the font in okular becomes blurry. With a value of "1" everything is fine. I tested it with the PDF and ODT backends and the latest git version.
Indeed this is an issue with Okular. It seems that the rasterization scaling is done relative to the wrong resolution.
*** This bug has been marked as a duplicate of bug 362856 ***