If supervisor used libel from a third party in an attempt to cause defamation during a performance appraisal, is this harassment?

Based on your question, it sounds like a third party said something unfavorable about you which your supervisor used to give you a poor performance appraisal. It sounds like you want to know if that could be considered unlawful discrimination. Based on those bare facts alone, it is hard to say. However, if your supervisor gave you a negative appraisal, or has harmed you in other ways, based on your membership in a legally recognized protected class, then you could have a claim. Examples of protected classes include: age, race, religion, national origin, sex, and pregnancy. If you think your supervisors actions were motivated by a discriminatory animus you should consult an attorney and provide them with additional details about your case.