According to this article and other sources, there are four defined stages of culture shock that you could have your character experience. You could write them in any order, but this is typically the order in which they occur in the real world.
1: The Honeymoon Stage
The character falls in love with the culture. She becomes infatuated with the language, people and food in her new surroundings, possibly even to the point of unhealthy obsession - think "otakus" and people who become overly obsessed with Japanese and anime culture in the real world.
Maybe Christina discovers that the surface folk have a unique cuisine or a particular traditional language, and she becomes obsessed with learning how to mimic that part of their culture to blend in with the surface dwellers. Or perhaps she discovers that they have a special system of rules or laws that differ from how the Wastelanders think, and she tries to figure out how that system works and convinces herself it's better than the one at home.
This stage may even cause her to clash with her fellow Wastelanders, who maybe see flaws and issues with the surface dwellers (are they secrertly cannibals? etc.) that she doesn't yet see because she's become enamored with their culture and way of life.
2: The Frustration Stage
This is the most difficult stage of culture shock. The character becomes fatigued by her inexperience with the culture - she doesn't understand their gestures, signs and language quirks, and it's exhausting and makes her angry and frustrated. She wishes she could understand these people better, but no matter how hard she tries she just can't get it right, and the culture seems even more foreign and alien than before.
Maybe in this stage, Christina has some internal conflict about how she fits into the surface dwellers and whether or not they will accept her, a Wastelander and a newcomer to their way of life. Perhaps her flaws in understanding their culture become more apparent in this stage, and make her insecure.
3: The Adjustment Stage
This typically occurs after the Frustration Stage. The character comes to terms with the culture a little more, and becomes more familiar with it. She becomes comfortable interacting with the surface dwellers and learning their strange customs, no matter how alien they seemed to her in the previous two stages. Navigation and social skills become easier, and friends and communities are established in the new culture.
Maybe in this stage Christina makes friends with a surface dweller for the first time, and they teach her more about their way of life.
4: The Acceptance Stage
This is usually the last stage, and represents the end of culture shock. The character is fully adapted to the culture, and has possibly even assimilated with it. She is comfortable and happy living among surface dwellers, and sees their way of life as similar or equal to her own Wastelander culture. It doesn't mean she fully understands them - it just means that she realizes complete understanding isn't necessary to function, and that she is familiar enough to have gotten over being frustrated or completely obsessed with them.