Green has been great this year but McLemore still looks bad, with a PER of less than 10. However, he does have all the tools and potentially has a lot of improvement in his future. I wouldn't call his season impressive though.
PER isn't the be all/end all. Especially when it contradicts McLemore's much improved shooting percentages and what is actually witnessed on the court (not lost defensively, playing with confidence, playing off of teammates, etc.). I encourage to watch him play and see if those PER numbers aren't deceiving.
Brandan Wright has the 2nd highest PER in the league at 29.96, which is more than James, Paul, Cousins, Curry, and so on.