Someone on the nix github came though with a work around
This one-liner fixed it for me:
environment.etc."xdg/menus/applications.menu".source = "${pkgs.kdePackages.plasma-workspace}/etc/xdg/menus/plasma-applications.menu";
Don’t forget to nixos-rebuild switch and kbuildsycoca6.
Adding it you your config fixes it only tested on wayfire but i bet it works on others.