。
She took the story as a joke.
Literal
She [topic-は] that story [object-を] joke [copula+quotative-だと] considered.
~だと考える ('to consider as / regard as') uses the quotative と to embed a judgement: 'thought to herself, this is X.' The pattern is the standard way to express categorizing or interpreting something — 失敗だと思う ('I think it's a failure'), 嘘だと感じた ('felt it was a lie'). 冗談 ('joke') here may refer to a humorous remark but also, in the right context, to something dismissable as not serious — 'she didn't take the story seriously' is a perfectly natural reading depending on the broader context.