The fonts in GTK/Gnome flatpak applications in the Fedora 35 KDE wayland session currently look like garbage (they are not antialiased) unless you run them without the wayland socket. This is due to xdg-desktop-portal-gtk being built incorrectly. You can fix this by installing xdg-desktop-portal-gnome and rebooting.
Step 1. sudo dnf install xdg-desktop-portal-gnome
Step 2. Reboot
Step 3. Verify that xdg-desktop-portal-gnome is running instead of xdg-desktop-portal-gtk
Fonts in flatpak'ed GTK/Gnome applications should look fine now.
See these bug reports for more details: