I’d be surprised if that isn’t already installed, given that LXQt in Whonix 18 is Qt-based and running on Wayland via the Wayland backend. Maybe the Monero Wallet GUI has a built-in Qt that was compiled without Wayland support.
This seems like the best option, although I think the arguments are backwards; wouldn’t you need export QT_QPA_PLATFORM=xcb; /monero-wallet-gui.AppImage? Or perhaps just QT_QPA_PLATFORM=xcb /monero-wallet-gui.AppImage (note the missing semicolon) if you don’t want to export the variable in the current terminal.