They're going to have an extremely uphill battle to fight if this is the basis of their legal argument. No reasonable person could understand the phrase, "Don't Be Evil," to mean, "Don't do things that I personally consider evil." For example, I very much doubt that a court is going to find that Customs and Border Protection is "evil," and thus any actions the plaintiffs took against Google's dealings with CBP were contractually mandated by the employment agreement.
To the extent that this is a headache for Google at all, it will serve as a great example as to why legalese and corporate-speak is so common in the U.S. Google tried to have a motto that, while legally ambiguous, expressed a sentiment about how they wanted to treat their users and act in the marketplace. There will be different opinions about how well Google lives up to the motto, but certainly at the time it was coined, that was the idea behind it. And now they're getting sued, essentially frivolously, over the language. If it causes them a problem, it's a lesson to other companies: avoid mottos that are subject to creative and motivated interpretation during litigation. Or to put it another way, don't have mottos.