Installing the server itself, the OS, or prerequisite software for the NMS or the NMS itself: off topic. This applies to troubleshooting any of the aforementioned activities as well.
Once the NMS itself is actually deployed and operational, this does get into a grey area.
Question about non-obvious error message you saw in your NMS? Probably off topic - contact the vendor (or community if it's open source). Is there a chance that someone here has seen the error? Yeah, sure, but going back to the looking glass question, this shouldn't be the default place to come and get operational support.
edit: If you've done your homework and have exhausted all other possible avenues, and/or you don't have a support contract, it could be considered on-topic.
Question about best practices for deploying an NMS? Probably on-topic, but the author should be careful so as to not ask the question in a way that leaves it "more open" to discussion. Here is a somewhat decent example.
See that your NMS supports SNMPv3? Don't know what that is? Definitely on topic.
Question on how to configure a Juniper/Cisco/Brocade/whatever for SNMPv3? Definitely on topic.
There are probably other good examples, but with respect to the linked question, I think it's easy to make the mistake of thinking that it's on topic if the OP has zero experience with Puppet or Ruby.