There is an issue on github for waybar repo: Segmentation Fault on start · Issue #3371 · Alexays/Waybar · GitHub. As mentioned in the issue, there has been a lot of commits since the last release (the one in nixpkgs): Segmentation Fault on start · Issue #3371 · Alexays/Waybar · GitHub. They really should just cut a new release so nixpkgs can update.
In the meantime, using the flake works! Let me know if you need help using the flake directly.