Is Othello a good, believing but naive man or is he quite the opposite?
Othello is a character who is fully human, with good qualities and character flaws. There is no suggestion in the play that I can recall that suggests he is in any way an evil person, and that is part of the tragedy of the play, isn't it?
Is he naive? He is certainly more naive than Iago, who is one of the most cunning and manipulative characters in literature. But his naivete is demonstrated through his trust in Iago and what seems to be a certain inexperience with love, not in any other arena. For example, a successful military leader cannot be naive in performing his military duties, particular a military leader who serves at the whim of kings and princes.
Would Iago have been able to do what he did if Othello were not naive about friendship and love? Probably not. But, to some degree, his naivete was what made his love for Desdemona so pure and what made the destruction of that love so sad.