I don't know who the worst fighter to ever win a belt is. Brock Lesnar, perhaps?
The worst middleweight champ ever is Chris Weidman. He got his title shot by beating Mark Munoz (respectable, but on the decline) and Demian Maia in a boring fight that Joe Rogan trashed him over. Seriously, listen to Joe's commentary in that fight; he vicious against both Weidman and Maia.
As champion, Weidman beat three Brazilian fighters who were all in their late thirties and had significant mileage on them by that point. Both of his wins over Anderson were weird, he barely got through Machida in an absolute war, and the version of Vitor he fought was dad-bod Vitor who had just come off of TRT. Rockhold was the first guy he fought that was his age, his size, and on the rise in the division, and Weidman got wrekt.
What has since followed has been a strange and somewhat sad career of him finding odd new ways to get finished in fights. Even though I hope he recovers well from his injury, his career in hindsight is nowhere near what many expected it would be after his first fight with Silva, and if Weidman were to retire today, I think Bisping's career as a whole would look much better in comparison.