Problem:

I can view the fonts in Ubuntu MATE but I can’t delete them using the default app MATE font viewer.

Solution:

To solve it, we’ll use a GUI app. You can install and use the default font manager used in Ubuntu: Font Manager.

Step by step:

  1. This step is only for Ubuntu MATE. Open the Terminal and install the app. Type: 
     sudo apt-get install font-manager

    Close the Terminal.

  2. Open the app, typing Super key (window key) type font manager
  3. Inside the app, search and select the font and delete it!

Source:

https://askubuntu.com/a/371320/822132