Othello centers on the battle between appearance and reality and it is this confusion that leads to misunderstanding and circumstantial evidence of wrongdoing. The evil (Iago)almost escape unscathed and the innocent suffer interminably - mainly Desdemona.
The suggestion that Desdemona and Cassio are "too good" defies human nature and what most people strive for and being willing victims suggests that they should have recognized the problem and therefore foreseen the events that followed.
Many of the people in the audiences of the day would have recognized Iago and his schemes and in fact some would have respected his abilities. Jealousy is an age-old demon and has caused many rifts and even wars (!) so then and now readers should be able to learn from the events that transpired. Shakespeare, effectively, warns his audiences to be wary of was is and what seems to be.
Othello unfortunately is consistently misled by Iago despite even Iago's wife trying to warn him and refuses to allow anyone to cast aspersions on Iago:
"My friend, thy husband, honest, honest Iago" (V.ii.155)
It cannot be Desdemona's fault, nor Cassio's, that Othello is allowing Iago to manipulate him in this manner.
Shakespeare is also perhaps warning wives to be careful of who they choose as friends. In Elizabethan times, women would not generally have had male friends and, perhaps there is some intention to suggest that Desdemona should have known better. She defied her father by marrying Othello and so
demonstrates the capacity to deceive men.
This is a short-sighted view but would have been a popular opinion in the day. Many women would have, and did, stand by their husbands no matter what shortcomings they (the husbands)had. Men on the other hand, did not have to suffer their wives and could have them sent to visit their "cousins" only to return on their husbands' command.
Hence, for Desdemona to have such trust in Othello would not be unusual and her trust in him, even after she has spoken to Emilia and feels something bad is about to happen,is unwavering. Desdemona's acceptance of her fate and even the fact that she blames herself for Othello's actions, not allowing anyone to blame Othello when she is asked who committed such an act:
"Nobody; I myself. Farewell! / Commend me to my kind lord" (V.ii.125-126)
are not the actions or words of a character who is "too good" but one who is dedicated and loyal to the end.