Watch the first fight again. Lesnar has Mir on the ground within 15 seconds of the fight starting and is administering the same kind of schoolyard bully ass-beating that ended the second fight until he accidentally socked him in the back of the head, which mercifully gets them stood up. Fight gets restarted, Lesnar knocks him back down and continues the beating, until he just stands there and lets Mir put him in a kneebar. So yeah, I'm pretty comfortable chalking that up to rookie mistakes. And as I said, the second fight speaks for itself. So where has Mir demonstrated the necessary skills to beat someone who's a physical freak like Lesnar?
As for the skillset, he has already beaten Brock using superior skill. You might say it was a rookie mistake, it might be. Saying he doesn't have the skillset is ignorant, it is there. Whether the skills are assisted by his increased strength training and are able to be used on Brock who can smother his opponents is going to be the determining factor.