No he isn't. Carwin's a big dude, but he's not as big as Lesnar. That'll be evident at the weigh-in.
It's not only Lesnar's strength that wins him fights. You don't win a Heavyweight title on Strength and strength alone in mma. He's got brilliant Wrestling skills, incredible takedown speed and then once he's got you down that's when the strength plays a part. I don't think Carwin will be able to handle Lesnar. In Carwin's 3 fights in the UFC, I've noticed a pattern. He starts slowly, looks very stiff in the striking, gets rocked with a big shot (even Neil Wain rocked him with a big right), manages to recover and then gets 1 punch through to win the fight. I don't see Brock giving Carwin time to recover, or time to get warmed up into the fight. I see it being over, with Brock laying down Ground and Pound before Carwin's had a chance to get settled.
Unless Carwin's worked incredibly hard on his striking, to improve his stiffness and defence of shots, I don't see anyway he can possibly win the fight.