Are there any language requirements for citizen applicants with low English skills?
Usually, universities set the requirements by native language and/or educational experience in English. If you have a bachelor's or PhD from a US university, they usually waive your language testing requirement because (1) you probably have already passed one many years ago and (2) if you're not good enough at English you probably would not have received your degree. 

But if you're just an American citizen, no. They don't usually waive TOEFL just because of your passport. There are many Americans out there who don't even speak English. Of course, if you're an American citizen whose native language is English, you would most certainly not be expected to tale the TOEFL.