Hi,
Kindly see the screenshot attached of Reaper DAW and the fonts showing up on it's menu. It is clearly a fixed-width font, and completely inappropriate (IMHO). I've been using Reaper on Lubuntu 19.08 and 18.04 for a long time before moving up to Mate 20.04, and never saw this font issue.
I have tried searching on the Reaper.fm support forums, but didn't see any such issue being reported. There was one about bad font rendering, with resolution being to copy libSwell.colortheme to the reaper directory, but that didn't help either. What gives ?
[Edit]
Installed Lubuntu 20.04 in a Virtualbox and reaper DAW in it, and the font rendering there is perfectly fine (not such big, bold, fixed-size fonts).
cheers.