Is there a way to make featherpad use the default system font?
It does if (1) you have the latest FeatherPad and (2) the default system font has a fixed pitch (monospace). Just uncheck Preferences -> Text -> Remember font.
That is not very obvious. It should at least be on the Options->Font page.
A text editor needs a special kind of font called “monospace”. Making things “obvious” to all isn’t my goal.