Does feeling joy for high grades and anger for low grades constitute a blemish on sincerity for a student with good intentions who strives for excellent grades and GPAs?

Rejoicing over a good deed and grieving over a bad deed does not diminish sincerity; it is a natural human response. Allah the Almighty has clarified that what is displeasing to a person is a bad deed that grieves him, and a good deed delights him. This does not affect your sincerity if your intention is pure. Indeed, Umar ibn al-Khattab, may Allah be pleased with him, rejoiced when he wished that his son Abdullah had said that the palm tree is the tree that resembles the believer, which indicates that a person's joy at success does not cause harm.