Are comments on application folders accessible to students? If a student applies to a university in the United States, can they legally access all comments written on their application (e.g. as a consequence of the FERPA)?
If a student is admitted to the university and ultimately enrolls then yes, under FERPA they have the right to access any educational records about them that have been retained by the university.  However, this doesn't include confidential recommendations in cases where the student explicitly waived the right to review the document.  It also doesn't include the records of students who were denied admission or who were admitted and didn't enroll (FERPA rights begin when a student enrolls at the university.)  

In the last few years it has become common for students to request this information.  There have been articles about this in the campus newspapers at many prominent universities.  In response to this trend some universities have changed their policies on retention of data from the admissions process (e.g. you can destroy any notes at the point when a student is admitted and then there won't be any record for the student to access.)  

Keep in mind that the vast majority of students in the US attend institutions where admission is either completely open or is based purely on high school grades, standardized test scores, etc.  In these cases there typically aren't any "comments" on the student's application.  

See these articles:

http://www.browndailyherald.com/2015/04/22/ferpa-requests-yield-limited-access-files/

http://www.stanforddaily.com/2015/03/09/first-students-gain-access-to-their-admissions-files-through-ferpa-provision/