ObsidianMD

You can also do this with any theme if you setup a CSS Snippet:

// This is the one that works for me
.markdown-source-view.mod-cm6 .cm-scroller {
    font-family: monospace;
}

// You may also want to do this one
.markdown-source-view {
    font-family: monospace;
}

Then reading view will use whatever font you've configured in the settings.