No doubt Portnoy plateaued during the Six Degrees/Train of Thought era, but to call Portnoy's playing 'leagues' below the likes of Bruford, Peart, and Mangini is a little ignorant. What MP lacks in technical prowess, he makes up for with creativity, but MP in his peak was one of the best rock drummers in the world. What make Bruford and Peart less mechanical sounding like Mangini is their sense of groove and of course jazz influence, which I hear none from MM. Portnoy has that in him a little (as do Petrucci/Myung/Rudess)
I personally disagree with calling him one of the best Rock drummers in the world, but at the same time, I don't like the genre and I have no idea which drummers count as Rock drummers and which ones don't
I won't deny his strong influence in the Prog scene, though (Blake Richardson is one of my favourite drummers of all time, and he's influenced by MP).
As far as I can hear, both Bruford and Peart (especially Bruford) had a better navigation through more complex rhythms, with more precision (compare Portnoy playing Larks Tongues and Tom Sawyer, there's many things he skips on those songs).
As for the Jazz claim, Portnoy improvisation and swing aren't Jazz-y (LTE jams, Liquid Trio Experiment's Jazz Odyssey). As for MM, you can hear a lot of Jazz in this video shared some messages ago
https://www.youtube.com/watch?v=M6y4ipRjDME. While it's not, idk, '60s-ish swing, it's not away at all from the likes of Chad Wackerman, for example.