No, an author/maintainer is free to do as they see fit. If they don't want to merge your PR, that is their choice.
If you don't agree with that choice, you are free to clone/fork the project and merge the PR there. No one -- even the author/maintainer -- is stopping you and they have, in fact, granted you the permission to do exactly that (via their choice of an OSS license).
How do you 'automatically merge' a comment where a user voices a popular concern? What about a merge request that needs to be rebased? What about a merge request that requires deep knowledge of the project to understand if it would break something critical, or introduce security flaws?
A nation state could submit a merge request to the Linux kernel that is a blatant backdoor and override maintainers by just shear numbers of 'supporters'? This is a very bad, no, terrible idea.
Code merges should not be an open democracy with no opportunity for oversight.
