Is it possible to boot to the terminal? I've tried to look for a grub file but I could not find it. Also /boot/firmware/config.txt does not seem to have such option.
Thanks to Rohith Madhavan and AceFace on Ubuntu-mate bitbucket issues:
You can boot to shell using the following commands:
systemctl set-default multi-user.target --force
systemctl disable lightdm.service --force
systemctl disable graphical.target --force
systemctl disable plymouth.service --force
Replace disable with enable to boot with GUI again.