I'm not going to get into the details, I already had this same argument in the "Greatest Roman Figure" thread of all places, it's better suited here.
It doesn't matter what Gratian should or shouldn't have done, or by who the Goths were mistreated and all the great things Valens did before the battle of Adrianople. What matters is, Valens was the emperor and he personally led the battle against the goths and lost it and managed to get himself killed in the process. In his capacity as Emperor, he had the power to alter the outcome, but he made one mistake after the other, first by provoking the Goths all the way until he led his troops into disaster. Valens is responsible for all the failures becasue he was emperor at the time.
All his good policies prior to the Battle of Adrianople only mean something if he was victorious or somehow avoided such a confrontation, but because he lost it, all his other accomplishments became obsolete. Going back and forth about Valens and making excuses for him is reading too much into histroy, just take the facts and you will find, based on the outcome of the battle, Valens is one of the worst Roman Emperors.