Eugénie was the loudest, and most powerful, proponent in the council of state for both the Mexican Adventure and preventative war with Prussia. If the former is successful, as opposed to IOTL, she'll be more sated, and Louis-Napoléon will likely be able to ignore her efforts, both politically and personally, to push him into war with Germany to an extent he couldn't IOTL.
Of course on the other hand, from my reading of Eugénie, a successful Mexico might also simply make her even more ardent about the 'German Issue' - "I was right about Mexico, so you know I'm right about those pesky Prussians."
Yeah. It's something that would take someone who knows more about her than I do to judge.