First of all, because of his high number of games played it's easy to think he's a seasoned vet been in the league for a long time, but let's all remember: Olli Maatta is still only 23 years old, has dealt with some injuries. and hasn't had a full off-season to train for years (two cups, and another summer with mandatory military service).
I feel like the only two groups of people who hold this opinion are A) "Yinzers" who always need a whipping boy or B) People who don't watch him play very much.
Olli has significantly improved both his skating speed and strength. He's not blazing fast, but he doesn't seem to have cement skates anymore. And he's not getting body'd off every puck.
His defensive game is sound. He can still get beat on the outside by a speedy player, but this is more due to his food speed, not any sort of inadequacy in his defensive play. His positioning is great, his stickwork is pretty solid. He doesn't have a very hard shot, but he's accurate with his slapshot and knows where to put it. His offensive IQ is pretty good, and he moves the puck pretty well. His decision-making can be suspect at times, but not many 23 year-old defensemen are going to be void of mental errors.
I have no idea how he can be "overrated". Literally no fans around the league think he's very good, and even a lot of Penguins fans trash on the guy. He's a solid middle pairing puck-mover who needs to continue working on his skating.