It sounds as if this comes from a school or college report and in this context the phrase "your sincere attitude" probably means "your conscientious attitude", ( = you do your homework, pay attention, are interested in the subject .. ), but "sincere" is not quite the right word to use.

Sincere can be used as True,genuine,not deceitful and honest etc. But in this context it's used as serious and concerned about work. Hope it helps