In theory: yes. In practice: mitmproxy is written in Python so there will be a delay because of the language not being all that fast. When you're visiting web pages with hundreds of small delays, you'll notice.

That said, for many people who care about this stuff, this could be an option. There's nothing preventing you from doing this technically speaking.

There's a small risk of triggering subresource integrity checks when rewriting Javascript files, but you can probably rewrite the hashes to fix that problem if it comes up in practice.