Hey guys, I have been playing with font combos in my theme. I took this formula form Pop!_OS and it really does make a nice change to readability and the look of everything. I thought it may be something worth considering that would be fairly easy to change and may make a big difference in appearance of the distro in general.
Window Titles: Fira Sans SemiBold 10
Interface: Fira Sans Book 10
Documents: Roboto Slab Regular 11
Monospace: Fira Mono Regular 11
So how would a novice try? I found the fonts online and installed using Mate Font Viewer. I opened Mate Tweak/Windows/Fonts. I see Window Titles and Documents. I don't see Interface or Monospace.
I think that the default (Ubuntu) is legible enough. Personally, I like Cabin as san-serif and Iosveka for monospace better, but Fira Sans is good too!