That depends on whether you're looking at it as if he's overrated overall or if you're pinpointing a specific moment in time...since you know fighters actually decline over time and don't remain at their pinnacle forever. However, until a fighter actually loses many people will continue to rate them as if they're still in their prime. This is what causes a fighter to be overrated.
GSP was being overrated by a lot of people as they acted like he was unbeatable after he came back from his ACL surgery, but if you really were paying attention you'd notice that he was much slower and not as explosive as he had previously been before his injury.
Silva lost about 30 mins out of 35 mins of fight time against Chael Sonnen, was rocked a couple times standing in their first fight but people still acted like he was in his prime a couple years later when he fought Weidman despite being 38 years of age. At that specific time he was being overrated, but his career as a whole and his accomplishments weren't overrated.
Barao is different, he got totally outclassed while in his prime by a better fighter. I look at what happened to him like I look at what happened to JDS in JDS/Cain II.