Lillard is not a terrible defensive player and does put effort into it. He is also not is 6'6" tall with a 7'0" wingspan. He can be overmatched, but he has gotten stronger and in the last couple of years has been able to use that strength like Jrue Holiday or Chris Paul to muscle taller players away from the basket or prevent them from getting by him off the dribble.
Having said that, he also plays a lot of minutes and his style causes him to exert a lot of energy on offense, especially since he is the number 1 option on offense. Holiday and Paul aren't the number one options on their team, so they can focus on offense. A different style of offense that doesn't rely on isolating Dame so he can go one on one all the time will probably result in Dame's scoring going down, but his defense would improve.