Teradata Viewpoint is Teradata's strategic and innovative SOV (single operational view) for Teradata DB, Aster, and HDP Hadoop systems management and monitoring that enables Teradata's Unified Data Architecture (UDA).
They don't impact performance, but may impact the ability of logging on new sessions.
As a Parsing Engine allows up to 120 sessions and you probably got the default 2 PEs per node you will have 8*120=960 session available on your system. If you start running out of session you should abort prolonged idle end user sessions (can be automatically done using Viewpoint).
When a session is not disconnected the logoff is either not programmed correctly in the client (and should be fixed) or the reporting tool might use a session pool (and then it's by design)