You can chenge it in the options, under Appearance there's Editor Font Family and Editor Monospace Font Family so you can adjust both. Options are under Tools>Options on Windows, on Apple I think it's Joplin>Preferences?
Can you share a screenshot? This is not the normal behaviour of Joplin. Is it possible that you have a userstyle.css or userchrome.css that is doing this by accident?
Just a comment, but italics really shouldn't be the default that is applied universally to all languages in a multilingual piece of software. The reason is that there are many alphabets/scripts where italic script simply does not exist. The prime example are the so called CJK languages, i.e. Chinese, Japanese, and Korean. If you force italics on Chinese characters, they will still render, but the complex ones may become very difficult to read. If you have a look at any proper text written in one of the three languages, you will find that no italics is used there at all.
I agree but this also wasn't something written by the Joplin team, solarized.css is a codemirror theme downloaded with the rest of the codemirror node package as you can see below.
Personally I don't think it makes sense for it to be italics but that is what is in the theme, it is pretty easy to override though.
I guess nobody has really complained about it before because relatively few people use it so it hasn't come up in conversation: