He's making a generalization. He's not saying every HW is mentally weak, just that there is tendency. It's not a black, and white.Its obviously just Mir making a dig at Brock Lesnar's lack of fortitude.
Please tell me how anyone in history is more mentally strong than Antonio Rodrigo Noguiera.
Maybe "mental weaknesses" is a gross exaggeration. I think general big guy laziness, and relying on natural size attributes is what is mostly at play here.