[2021] Help! Most of the characters in the game look like nightmare monsters!

Has to be an installation or video driver issue, AFAICS. The minimum requirement under Linux is the HD 7870, so I'm afraid your HD 7770 is a little below that. Whether that's the cause I'm not sure.

Please attach your Documents/Paradox/CK3/pdx_settings.txt .
From your Documents/Paradox/CK3/logs/ folder, attach system.log , error.log


Also in Steam please click on Steam-Help-System Info, and paste everything there into a text file and attach that here.