Matthew takes forward the characterisation of Peter as the disciple who is scandalized by the idea of "Christ crucified" (1 Cor. 1:23), and he underlines and enhances the same language of skandalon and skandalizomai ("stumbling block", "to fall away"). Far from whitewashing the disciples, Matthew in fact proves to be a strong reader of Mark, understanding and elaborating on his presentation. One of the reasons that we fail to see this is our over-reliance on redaction-criticism, and our tendency only to pay attention to the parts where Matthew differs from Mark; we do not take seriously the elements that Matthew takes forward and underscores in Mark.
For those who would like to follow up the discussion on this point, I have an article available on the issues. It was published in 2006, but I am happy now to make it available online in toto (PDF):
"The Rock on Rocky Ground: Matthew, Mark and Peter as Skandalon," in Philip McCosker (ed.), What Is It That the Scripture Says?: Essays in Biblical Interpretation, Translation, And Reception in Honour of Henry Wansbrough Osb (Library of New Testament Studies; London & New York: Continuum, 2006): 61-73
The essay has been available in the above Festschrift for Henry Wansbrough for the last three years, but I am happy now to make it available free for all on the internet.