I can't see how you would underestimate it. Cormier out grappled Josh Barnett, who's got a fantastic ground game. Personally, I think Cormier would finish Lesnar inside of 2 rounds.
this would be a great match up. Cormier has the better hands but Lesnar has a large reach and possibly strength advantage. I would probably see DC winning by UD after out pointing Brock with strikes. I don't think he could finish Lesnar though.