Here's FixedFont is being mapped to GtkFixed: https://github.com/qt/qtbase/blob/c1e1d133c47f6cbeee045dcec0dfe7c68d5bef34/src/plugins/platformthemes/gtk3/qgtk3interface.cpp#L546
Looking at the description of GtkFixed widget, it's a completely wrong widget: https://docs.gtk.org/gtk3/class.Fixed.html
See how libvte does that, it creates a custom widget with GTK_STYLE_CLASS_MONOSPACE CSS class (I guess creating a GtkWidget instance and adding the class to it will be enough for Qt use-case): https://gitlab.gnome.org/GNOME/vte/-/blob/master/src/widget.cc#L651-652

I've got a report that an application uses a sans-serif font instead of the monospace font, so the QFontInfo::fixedPitch check seem to not really work, not in all cases at least.