Theoretically, your liability is the same because the First Amendment is what absolves you of liability for someone else's speech. Section 230 provides an avenue for early dismissal in such a case if you get sued; without Section 230, you'll risk having to fight the lawsuit on the merits, which will require spending more time (more fees).

I'd probably like the upvote itself to be considered "speech". The practical effect of upvoting is to endorse, together with the site's moderators and algorithm-curators, the comment to be shown to a wider audience.

Along those lines then then an upvote i.e. endorsement would be protected, up to any point where it violated one of the free speech exceptions, e.g. incitement.