Could you run htop to see how much memory is left?
I'm considering using this with my 2GB version if it won't take half of the memory. EDIT:
From what I can see in here, installing the Mate desktop on Ubuntu x64 will take about 900MB.
That's a lot! My 32 bit Raspbian takes less than 400MB.
As a much lighter-weight alternative, try https://ubuntu-mate.org/ for RPi2/3 --it uses a lot less resources to run, yet comes packed with the Mate-Desktop (pronouced MAH-tay), which was branched from Gnome 2.32 to become it's own desktop, several years ago now. It's personally my favorite distro, which I run on x86/x64 and ARM32/ARM64, and I have it installed and running on 3 embedded system and 6 VMs.
The procedure to install it is more in line with community standards, rather than using an all-in-one imager/eraser tool, but the results tend to be better because the partitioning is direct to the SD card rather than within an encapsulated FAT-based file system-object-file. The results are a fast clean OS with a ton of capability.