An addition to MDMower's answer above about Manual or Automatic Installation.(*)
On Ubuntu 16.04.5 LTS with hwe-16.04-edge kernels and Cinnamon as DE, I experienced unexpected font behavior upon installation/upgrade of libfontconfig1 and its dependency fontconfig-config. Suddenly, I had weird and thin and barely legible mono-space fonts and other "ugly" fonts in many application's menu bars, as well as in terminal windows and many other places throughout my applications.
In case you encounter this issue: There seems to be no working fix at runtime for this. No point wasting your time searching for a setting to correct in any of Cinnamon's or Gnome's or FontConfig's or your Terminal's preferences (e.g. gconf-editor/gconftool, gnome-tweak-tool, fc-*). That won't have any effect on the "broken" fonts. It seems the update of Fontconfig-Config somehow resets known/cached references to certain fonts, and some fallback fonts are being rendered instead, while still referring to the fonts by their original name.
Simply restart your system, and everything is back to normal. Even just logging out and back in might suffice.
(*) This was supposed to be an addition or comment to above answer by @MDMower, but insufficient StackExchange "reputation" prohibits me from posting it as a comments. My edit was rejected (partially rightfully so), as well, hence this spammy "answer" instead. Sorry. I would be glad if someone with enough "power" might move this "answer" to be a comment, where it belongs. Thanks!