When there are no side effects and no in-container secrets and the hosting is free or reasonably limited to prevent abusers, ideally yes.
Outside that, heck no, that'd be crazy. You're allowing randos to run arbitrary code on your budget. Locking it down until it's reviewed is like step 1, they can validate locally until then.
The issue is that now the pull requests don't get tested at all. I have to manually, locally, get all the commits, make a branch on the main repository with them, and then the actions run.
Challenge there is that if the pr changes, it's a bit clunky to retrigger the CI (you have to remove, then re-add).
I guess you could also do this with comments - can you trigger a workflow based on a specific comment being added from a specific user?
My prior is that GitHub is pretty competent in understanding malicious PRs to open source repos, and wouldn’t penalize the repo owner without other evidence of wrongdoing.