This seems to come up on various Linux distros from time to time. For example: https://github.com/zaproxy/zaproxy/issues/3051
The following suggestions may let you work past the issue.
- Try starting ZAP from the command line instead of starting it from a shortcut (such as a Plasma/KDE button, dock icon, etc.)
- Try adding the following to
/etc/environment:
_JAVA_OPTIONS='-Dawt.useSystemAAFontSettings=on'