Yeah, that's why the smallest HW in Randy did the best against Lesnar.
Couture has some of the best wrestling of any MMA fighter. It allowed him to survive against Lesnar. He was still losing the fight in all regards except possibly octagon control. He just wasn't getting his ass whooped.
Someone his size without amazing wrestling is going to get beat down by Lesnar. There is only one other fighter that has really good wrestling in the HW division and you think he is garbage...
Mir can work on his wrestling and it will help him but he isn't going to get to a level even close to those fighters. Bulking up and increasing his strength is his best and most readily available tool in giving him a chance of winning the title back.