That's the point. They're supposed to contrast with Karna. Arjuna is supposed to be that way because he is supposed to embody good and humility, traits that people expect of him for being a great hero which is the point of his character. Not everyone has to be walking Visual Kei
Furthermore, simple =/= bad. Arjuna's design is very elegant and actually has many references to the common depiction of Arjuna in the Mahabhrata.