Badly written, or badly designed? Bad code, to me, would be an unrolled for-loop, or a for-loop where a foreach loop would do better (though now, we're getting in the realm of nit-picking). A PR is waaaaay too late to bring up architectural/design improvements (unless it is a WIP PR opened expressly for discussing the approach).
I very rarely see bad code in PR's unless it is from a Junior programmer, and even then, it's usually because they don't know better. Even then, I choose to trust them to do the right thing. In most environments, you can even review the code AFTER it has been merged. So, if they don't address the issue, I can still review the code and let them know that I expect to see a follow-up PR addressing it.
I very rarely see bad code in PR's unless it is from a Junior programmer, and even then, it's usually because they don't know better. Even then, I choose to trust them to do the right thing. In most environments, you can even review the code AFTER it has been merged. So, if they don't address the issue, I can still review the code and let them know that I expect to see a follow-up PR addressing it.