The Debian image on DigitalOcean has GTK2 and other GUI libraries installed by default. I noticed this by chance because there were fontconfig updates available for my droplet.
The apt dependency chain pulling in GTK is this:
# aptitude why libgtk2.0-0
i mutt Depends libgpgme11 (>= 1.2.0)
i A libgpgme11 Depends gnupg2 (> 2.0.4)
i A gnupg2 Depends gnupg-agent (= 2.0.26-6)
i A gnupg-agent Depends pinentry-gtk2 | pinentry-curses | pinentry
i A pinentry-gtk2 Depends libgtk2.0-0 (>= 2.10.0)
If I uninstall pinentry-gtk2 and install pinentry-curses instead, 41 packages get deleted, freeing 65.7MB of disk space. But it's not about freeing a couple megabytes; it's just none of those packages are useful in a server, they are only useful in a Linux desktop environment.
I have no opinion one way or the other on having mutt, but if you're going to install mutt in the image, you should also install pinentry-curses to keep apt from automatically bringing pinentry-gtk2 instead.