Is there a stigma of assigning the title TA to an undergraduate?
A teaching assistantship (TA) is a formal job category at most universities, which implies that you are being paid a specific wage for teaching duties while a graduate student. Consequently, you would be misrepresenting your work on the CV if you called yourself a teaching assistant, because you are not formally a graduate student.

At the university level, another issue may be that while many undergraduates do participate in teaching responsibilities at many schools, this is often in the form of tutoring programs and informal arrangements. Part of the reason for this is that many universities want to be able to claim a higher percentage of teaching duties are done by faculty than is really the case. Calling undergraduates graders and tutors helps to shift off the balance, since they are not officially "teaching staff," and therefore wouldn't be included in such time accounting.