Why not just toggle off the viewer? It sounds like you just want plain text, which is what the editor does. You can use this button 
to toggle the viewer/editor mode (it's in the top right corner).
If you don't like the styling in the editor (understandable), you just need to use a small amount of css to change it to be whatever you want. You can see the needed css here. You add this to your userchrome.css (Which can be accessed through Tools -> Options -> Appearance -> Show advanced).
I know this isn't ideal for you, but Joplin can't support a button for every styling option that users want (since that is basically limitless and is pointless when CSS exists and is so much more powerful).
That said, there are efforts in progress to make user styling easier (likely two clicks instead of one). So you're just a bit little to early into adopting Joplin to be able to take advantage of that.
And an example for posterity
