Does feeling pleased with one’s grades contradict sincerity of intention in seeking knowledge?

It appears, in sha’ Allaah, that this does not undermine sincerity of intention, because it is natural for a person to feel happy about something good and be upset by something bad. Allaah has explained that things which do not agree with man are called bad things (sayi’ah), so that should upset him, and that the good things (hasanah) should make him feel happy. 

This does not affect your sincerity of intention if, as you say, your intention is good. But if your concern is only your grades or certificate, then this is another matter. Look at ‘Abd-Allaah ibn ‘Umar ibn al-Khattaab (may Allaah be pleased with him), when the Prophet (peace and blessings of Allaah be upon him) posed a question to his companions and said, “Among the trees there is one which resembles the believer.” The Sahaabah (may Allaah be pleased with them) started guessing the names of all kind of trees, guessing the names of desert trees. Ibn ‘Umar said: “I thought that it was the date-palm, but I was young and I did not want to speak.” (narrated by al-Bukhaari, Kitaab al-‘Ilm). 

‘Umar (may Allaah be pleased with him) said to his son, “I wish that you had said it.” This indicates that there is nothing wrong with a person feeling happy when he does well and the like.