Your SQL Server can show critical status in SolarWinds if one of the metrics breaches the default threshold.
For example, i have a SQL Server Instance monitored by SolarWinds but is showing critical because the Lock Requests/sec is showing Red.
This would then send the top level of the monitor to red.
I believe you can adjust these so that it doesn't show the worst status, so would just be a warning status if you have a critical metric such as locks or latches.
If you are monitoring a SQL Server cluster you should be monitoring the SQL Server Instance Cluster Resource address and not monitoring SQL Server via individual nodes.
This is so that if your cluster fails over to the other node the IP address of the instance resources within the cluster manager will never change, if you monitor the Instance specific to the node then when it fails over it will say SQL Server is down because its not running on the individual node your monitoring.
I hope that covers your query.