If adopted children make up 2% of the population, then they are only "under represented" in law schools with close to 0 adopted students, so I doubt that it qualifies as any sort of extra consideration by itself. It's worthy of mentioning if it is/was a hardship for you of some sort, or if it gave you some sort of unique insight you can bring to the class.
I know several people who were adopted, and I really only see it providing unique insights into society in the instances where the parents and adopted child are racially different. It's just my opinion, of course, so if you can prove it wrong, please do.