Lesner would have destroyed him. He wasn't a great mma fighter, but he was a legit NCAA wrestling champion and was strong as hell. Kimbo was never anything more than a brawler and was never at the level to even compete at the lower level of the UFC. Lesnar would have taken him down and brutalized him ground and pound until the ref had to pull him off.
Roy Nelson took him down with ease and beat him with a crucifix position by round 2. Lesnar would end the fight sooner