Ask HN: How to deal with low quality merge requests?

We have a recurring problem with badly formatted code with stupid variable names, disgusting doc blocks, etc. One developer in particular tries to clean up his code line by line, wasting reviewers' time.

We're supposed to be in constant crunch mode, so coding guidelines are rarely written (they do exist however), and most office conventions are shared in verbal communication.

If the reviewers don't report problems on each and every line ("unneeded empty line", "clear up type hint", etc.), it remains inconsistent, ugly, or buggy.

I wonder how experienced engineers help their peers understand this problem better.