Ehh...saying Lesnar was "good" is being a bit generous. More than anything, he was overwhelming. He was a huge guy who came out like a bat out of hell, who could also take you down. The fight with Randy Couture just happened at the right time. Couture was at the tail end of his career...and honestly, Randy never had a stellar record anyway (though he was fighting top 5 fighters almost his entire career).
But once people figured out how to stop Brock's takedowns and make him box, his lack of skill on an MMA level was totally exposed. He could still punch hard, but his technique was bad. His head movement was bad. His footwork was bad. After surviving the first 60 seconds and stuffing the takedown, he basically became an amateur.