I get you now, when you said "mangled" I was assuming you meant that data itself was getting lost in the process somehow, like if you were to paste into Word or something and you lost formatting marks, indents, spacing etc. rather than it just being a display issue.
Technically I guess it is possible that someone could just make a very simple plugin that could apply that simple CSS which would literally be what you want, a tickbox to turn "plain" formatting on for the markdown editor.
There is a topic where people were discussing the virtues and drawbacks of a number of other note taking apps (primarily trilium) and one of the things I did like was the ability to set the "type" of an entire note which would accomplish the same goal by preventing the editor from changing font sizes etc. rather than surrounding the note in backticks.
- Being able to manually set note types - Joplin sort of has this already in that the "Markup" field can be either Markdown or HTML but it isn't something that can be set manually. I mostly would like to see it as a way to protect any HTML formatted notes from the markdown editor as well as being able to set entire "code" type notes that are excluded from formatting but can be distinguished at a glance.