Smarts does not monitor STPNodes. STPNodes are objects that Smarts creates, which are then used to create network connections. The presence of STPNodes in topology indicates that STP was used in order to establish connectivity between network elements in that topology.
An STPNode will store information such as Designated Root, Designated Port and Designated Bridge, which will then be utilized, for example, to create trunk connections between switches participating in STP connections.
Monitoring, alerting and Root Cause Analysis is available for those connections but not for STPNodes themselves.