Justices Ruth Bader Ginsburg and Samuel Alito take turns asking Halligan what the words after the semicolon mean, with Ginsburg finally challenging Halligan by saying: “You said that—that Young’s position is ‘most favored nation.’ Well, yours is least favored nation, right?”
[Justice Elena] Kagan, who has been a one-justice Tasmanian devil all morning, concludes by informing Halligan: “What we know about the PDA is that it was supposed to be about removing stereotypes of pregnant women as marginal workers. It was supposed to be about ensuring that they wouldn’t be unfairly excluded from the workplace. And what you are saying is that there’s a policy that accommodates some workers, but puts all pregnant women on one side of the line.” This is why pro-life and pro-choice groups and most women’s groups have crossed the ideological divide to support Young in this case: The PDA was intended to prevent women workers from being forced to choose between their jobs and their babies.
The author likely is meaning to say that Kagan has been asking hard questions all day, on many different topics.
What many Americans think of when they hear "Tasmanian Devil", is the character from Looney Tunes. He would spin really fast in a circle. Just like Kagan would spin from one subject/topic to the next. And obviously the Tasmanian Devil isn't a cute, fluffy creature, but a harsh, angry beast. Thus Kagan wasn't asking easy, obvious questions, but dared to ask hard ones.
It is a simple metaphor.
The reference to Tasmanian Devil has to do with taking on the characteristics of a Tasmanian Devil all morning. The mention of the PDA case is not germane to the metaphor.
Those characteristics are listed in the introductory section of the Wikipedia article you checked.
Since not many English speakers have seen a Tasmanian devil, the example that comes to mind most readily is the cartoon Tasmanian devil, Taz of Warner Brothers, whose main characteristic is ferocious whirlwind hyperactivity, as in one of Taz's first appearances (1957)