Hi,
Just as the title says: I installed MATE in Ubuntu server and after login in, GNOME started.
I logged out and there is no button or dropdown to select MATE. Isn’t that weird?
I even checked the command history to make sure I did type ‘sudo apt install ubuntu-mate-desktop’ and not the GNOME command.
It’s just bizarre. I’m thinking about reinstalling the whole thing.
Did you restart, as well?
Yes, after I didn’t see any changes when login in, I restarted. I was reading the tutorial and probably forgot to enable lightdm during the install steps. But that didn’t make a difference either. It’s a VM so I’m rebuilding it.
Cause I’m dumb! LOL I wanted the server ISO with MATE on top. For learning, I’m just going with Ubuntu Mate like you said. 
Don’t worry so am I 
Glad I could be of help.