Even though there are indications that it was done differently in Lois and Clark (and in Smallville for that matter) it is my pet theory that Clark's adoption was perfectly legal. If Martha and Jonathan declared Clark a foundling (which he techinically was) and adopted him as such (there are specific legal steps in getting foundlings birth certificates and Social Security Numbers) then there is no question about his citizenship in my mind.
Hehehe!
I found proof. In season one of Smallville at one point Chloe is looking into Clark's adoption and the reason for adoption is listed as "Found alone. Unable to locate parents" Clark was declared a foundling and that is how the Kent's adopted him.
So, may not apply to Lois and Clark exactly, but in Smallville, Clark is a legal US citizen, adopted legally.
