Copying /usr/share/feedbackd/themes/default.json to $HOME/.config/feedbackd/themes/default.json like mentioned by @Pelecanus and rebooting the PinePhone disabled vibrations on Mobian, I did not even need to edit the file.
However switching from the full to quiet profile does not make vibration work so the theme does not seem to be working as intended.
WARNING: If you copy /usr/share/feedbackd/themes/pine64,pinephone.json to $HOME/.config/feedbackd/themes/default.json and reboot you may get an issue like I had, the profile switching icon was grayed on Phosh and there was a black screen when starting the terminal so I had to connect the device to my computer with a USB cable and SSH to revert the changes.
In my case the best solution was to add "blacklist gpio_vibra" in a file like /etc/modprobe.d/blacklist.conf as mentioned by @iwakura_lain to keep vibration disabled after boot. If we want to enable vibration temporarily after boot, we can do it with the command "sudo modprobe gpio_vibra" and disable it again with the command "sudo modprobe -r gpio_vibra".
This gives flexibility to use the full profile to have sound notifications with or without vibrations, use the quiet profile for vibrations only or the silent profile for no vibrations and no sound notifications.
However switching from the full to quiet profile does not make vibration work so the theme does not seem to be working as intended.
WARNING: If you copy /usr/share/feedbackd/themes/pine64,pinephone.json to $HOME/.config/feedbackd/themes/default.json and reboot you may get an issue like I had, the profile switching icon was grayed on Phosh and there was a black screen when starting the terminal so I had to connect the device to my computer with a USB cable and SSH to revert the changes.
In my case the best solution was to add "blacklist gpio_vibra" in a file like /etc/modprobe.d/blacklist.conf as mentioned by @iwakura_lain to keep vibration disabled after boot. If we want to enable vibration temporarily after boot, we can do it with the command "sudo modprobe gpio_vibra" and disable it again with the command "sudo modprobe -r gpio_vibra".
This gives flexibility to use the full profile to have sound notifications with or without vibrations, use the quiet profile for vibrations only or the silent profile for no vibrations and no sound notifications.