If you think a specific question was closed unnecessarily, you can do those thing :
- Edit your question to add explanation of why it shouldn't have been closed.
- Flag the question for moderator attention with a message explainning why you think it was badly moderated.
- Open a question here to discuss about it. If the question got closed and it shouldn't, you should be able to get it re-open fast. Example.
Let's consider that all the option above don't exist, what would it bring to down-vote a moderator ? In my own opinion, the moderator that would have the most down-vote would be the one that close the most subjective topic and the one that are the most active.
Let's consider that the down-vote metric is useful. Why would people down-vote a moderator ? Either because the decision was bad or because they are unhappy about it. In one case, the down-vote was used for what it's for and in the other case, it wasn't. It's not because people are unhappy about a decision, that is was a bad one, but they will still down-vote the moderator.
Overall, I think it's just a bad idea. It won't bring anything useful to the website.