Same problem here on Linux. It seems to be bypassing Freetype. Nearly unreadable on my 1080p screen. It may be fast and featureful, but the appearance of the GUI and text are currently pretty bad.
I remember seeing this in iTerm, but it was fixed by changing the subpixel hinting mode. Is there an option for this in kitty? I couldn't find one in the config, unless it has a different name.