If the PR is technically correct, just violates coding style or smells a bit like of hacky trash, I merge it anyways. As long as it's not breaking the build, functionality, or bisectability, just go with it. You can always chase it with a cleanup commit of your own.
It's up to the PR's author to decide if they care enough to do better next time. By accepting their work and landing the change you've demonstrated an appreciation for their effort with open arms, and I feel this makes it more likely they will contribute in the future and care about doing better.
Maybe make a comment to that effect before merging as-is, creating the opportunity to clean it up themselves first.
If it's coming from a regular contributor I'm far less charitable, this approach I mostly reserve for first-time/infrequent contributors.
It's up to the PR's author to decide if they care enough to do better next time. By accepting their work and landing the change you've demonstrated an appreciation for their effort with open arms, and I feel this makes it more likely they will contribute in the future and care about doing better.
Maybe make a comment to that effect before merging as-is, creating the opportunity to clean it up themselves first.
If it's coming from a regular contributor I'm far less charitable, this approach I mostly reserve for first-time/infrequent contributors.