When you listen to the full quote of what he was saying, his perspective makes sense and isn't derogatory to MMA fighters. Title is kinda clickbait.
“That’s the thing about boxing,” Johnson said on his YouTube channel (h/t MMA News). “Mixed martial arts is the easiest sport to become champion in. When you fight boxing, it’s boxing. Every person you fight in boxing is going to be good in boxing... when you fight mixed martial arts, you’re gonna have people who have significant big holes in them, right? You have a guy like Alex Pereira ... 6-1 and he becomes a champion in mixed martial arts. You can have no wrestling experience and become a world champion. You look at Brock Lesnar, he became a world champion and he had no stand-up experience whatsoever.
“That’s why I feel MMA is the easiest to become a world champion ... you can have deficiencies and still become a champion,” he concluded. “In boxing, you can’t, you have to be a good boxer to become a world champion, you just have to.”