I am a new MATE user and i am amazed how good this is, after installation i wanted to remove some default apps which i dont use but there was no way see all the installed apps in one place, i had to search for each app which i wanted to remove.
I agree that would be nice. Just for reference
apt list --installed
Gets the job done.
That's an idea that's planned for "Software Boutique 2.0", but it's not ready yet.
For now, there's also GNOME Software and Synaptic that can list installed apps/packages. These can be installed from the More Software category in Software Boutique.