I don't know, I have mixed opinion on this. Ideally the workflow would be authors use `git commit --fixup` for fixup commits, and at some point when everything was approved, somebody does `git rebase -i --autosquash base`. Who's that somebody is unfortunately annoying because * if it's the PR author, the merger has to re-check this didn't introduce changes * if it's the merger, GitHub doesn't provide a way of closing the PR (at least not that I know of from the CLI side).
As I won't use the GitHub UI for merging, and would rather see PRs as "merged" than "closed", I still kinda like having the author to the squashing and just diff the before/after. Otherwise, letting the merger do it is probably best, unless the PR author is a power user.