I can see both sides to this.
Reporting the first user was warranted for breaking rule 2 (Be Civil).
However, reporting the second user for "downvoting a post they disagreed with" was not warranted.
The rules in that sub are specific, but, as the mod has stated, how you vote is not a part of any rule. It's a reminder of how you are expected to vote, but this should not be viewed as a rule that people can be reported for.
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In my opinion, that second report should have been seen as a misunderstanding/learning opportunity for the mod to reach out and clarify, and they should have given you a warning rather than jumping straight to a ban. If you had continued to report users for this same reason I could see a 1 day ban making sense.
From this post, it's not clear if you've had other interactions with the mod warning you against reporting for this reason. Considering how new your account is on here I would doubt it.
I disagree with the mod that you "started reporting anybody that disagreed with [you]" however, given your reasoning it's not hard to see that you were willing to do this and you could have easily continued down this path. This could be problematic especially given how easy it is to see how individual users vote in the fediverse (those votes are not private like they are on Reddit).
Overall I think this should have been more of a warning, and the mod could have handled the situation better. A 14 day ban would have easily been PTB material. The correction to a 1-day ban still feels excessive for what should have been a warning, but that in itself doesn't feel like this reaches the PTB level.
To clarify what I meant there, I'm not saying that they were willing to continue despite what the moderator said.
I meant that they would have continued to report other users for this reason until someone pointed out to them that this isn't really a rule violation.