Ties to the unfortunate Mama are now unraveling, leaving Cass or Cassie to stand on its own as a fresher sounding Cathy or Kate. But with so many lovely long forms, we'd advise keeping this as a nickname.
Cassandra "Cass" Cain aka Batgirl, character in Batman comics Cassandra "Cass," main character in "The Name of This Book Is Secret" by Pseudonymous Bosch Cassie "Cass" Kennington, character in movie "Have Dreams, Will Travel" Rose of Sharon "Cass" Cassidy, character in video game "Fallout: NewVegas"
"Cass," 1978 Australian movie
Cass Hamada, character in the 2014 movie Big Hero 6