To write a tutorial on how to install the Mate Desktop Environment on top of a fully functional Ubuntu Server would be a very easy thing to do.
To get everyone to the point that they could configure a Ubuntu Server and make everything work is quite another thing to consider.
For example, even though the installer for Ubuntu Server will find and configure a Ethernet or WiFi network for the purpose of installing the server software, after the installation is complete the network is no longer configured, and will have to be manually set up by gathering information and editing files from the command line.
Not only that, but someone will eventually find that after the server is installed that their network hardware has been disabled by the system, and they will need to know how to manually enable it again before the network can be manually configured from the command line.
Considering the vast array of systems that such a tutorial might be tried on, the variables are almost endless.
Obviously, if you can not get the network up and running, you can not download the Mate Desktop Environment software.
And there are many other things that will need to be fixed, cleaned up, and installed to have a fully functional Mate Desktop when installing on top of a Ubuntu Server. Even more to do beginning with 18.04 Bionic due to updates and changes made to the Mate DE.
The Ubuntu mini.iso does give the option of installing the Mate Desktop during the setup process. I've done so many times in a virtual machine. It's really quite easy to do. However, I am unable to test the mini.iso on my only i386 system because it does not have a CD-ROM drive and for some reason I can not successfully write the mini.iso to a USB drive.
Perhaps someone who has successfully installed Ubuntu Mate from the mini.iso on i386 hardware can weigh in.
If the mini.iso does indeed work in this fashion, it would be much simpler than trying to install a the Mate DE on top of a Ubuntu Server for most users who would want to do this.
Edit: Ubuntu 18.04 LTS Server is 64-bit only. The Ubuntu 18.04 Beta Network installer (mini.iso) does support i386 and can be downloaded by following the links below.
Download link with documentation: