Elayne taking over Cairhien is a good thing in terms of preparing for the Last Battle. Now the local nobles can concentrate on preparing for the Last battle and not waste their time and efforts only on political manipulations.

At the end of the day, the Cairhienins weren't conquered, their nobles chose Elayne themselves (OK, nudged by Rand and the knowledge of Elayne's armies and the dragons, but still...) and not much will change for them except more political stability.

With the Last battle coming, not to mention the Seanchan, any sane ruler would be ecstatic if presented with a powerful new weapon like the Dragons. It doesn't mean Elayne wants to conquer the world. Even if her two kingdoms remain intact after the LB, she'll have more than enough on her plate trying to keep Andor and Cairhien under her control to think about new conquests. With so much unclaimed land north of Andor, they can expand this way if they feel strong enough without having to attack any country.