Well, the rawness of sushi can be an issue, at least for humans - like dairy products, if it's not something you grew up with from infancy, you have a high probability of simply not being able to digest it as an adult. Also, if Atlee has been eating healthy for the last couple weeks, any American-made Asian food is going to be waaay too rich for her... in which case the burger will make her nauseous as hell too, but it does make sense for Peej to not realize that (kryptonian or not, she's still American culturally).