When I plugged my HiDPI display (Acer B276HUL) into a new MacBook Pro, the diplay resolution would only go up to 1920x1080, and the fonts looked jagged and terrible.
I fixed the resolution issue by buying a new, better USB-C to HDMI converter. After using it, 2560x1440 worked great, but the fonts were still not rendering correctly. Confusingly there are adapters that fit both sides but don't support full 2560x1440 resolution.
There are a huge number of forum posts online with people asking about this, and strangely Apple does a terrible job of helping them help themselves. It's almost as if Apple wants people to buy an Apple display instead of making their existing display work as well as it can.
The problem is that by default OS X uses a mistaken configuration for this display, something about how the different way pixels can be arranged in an LCD display. To fix the problem you generate a new configuration, turn off System Integrity Protection so you can make the new config available to the OS, then reboot. The process is described here.
After following these instructions fonts look perfect on the external display, and I turned System Integrity Protection back on. My faith in Apple has taken some damage though.