What you're forgetting is that when Helena's family was murdered when she was eight, the Gotham police sent her "condolence" cards that read "many happy returns." So hearing Barbara toasting the death of Mandragora, and then learning that Gordon and her colleagues are with law enforcement, could only bring all that back for her. When she says never to toast anyone's death, she's genuinely angry. Babs may be trying to be friendly, but she's failing miserably.