This is part of the sort of sorting out after the election to try and figure out just what happened, with a subtext of "Could Bernie Have Beaten Trump?" I do not have an answer to that. Certainly during the primaries he did much better in polls against Trump than HRC ever did, but then he was never exposed to the sort of relentless negative campaigning he would have faced if he had gotten the nomination. Certainly he would have done better than she did in some of the white working class areas of those Rust Belt states that flipped and put him over. But then Bernie may have done less well with some other groups, moderate GOP-leaning suburban women or maybe an even lower turnout among African-Americans, especially female ones. No way to parse all that in any definitive way.
But I did want to note that in those Rust Belt flip states (which should include Iowa along with PA, OH, MI, and WI), it is interesting to look at which counties flipped, and it is kind of amazing if one looks at the national map how few counties nationally actually did flip. But, unsurprisingly there were higher numbers of them in these states that themselves flipped.
So in PA and OH in particular, the "Bernie would have done better" claim looks strong. In PA the two counties that flipped were Erie and the one with Scranton in it, both old industrial areas that have had major job losses and been depressed for some time, with perceptions that much of this has been due to foreign imports and also having most of their local working class populations being white. Anyway, they flipped, and the state went with them, even if Clinton did better than Bernie might have in some of those Philly suburbs.
Somewhat similar story in OH, with the county flipping the most being Turnbull, going from 5% for Obama in 2016 to 22% for Trump, Turnbull being where deeply depressed old steel town Youngstown is located. Also, the depressed white working class suburbs west of Cleveland in Lorain county flipped.
Michigan is a little harder to tell.[Added later: It looks like MI is sort of between the PA and OH on one side and WI and IA on the other. So, there was some shifting in counties near Detroit, looking like the working class types shifting to GOP, but there were also a bunch of rural counties in northern MI that shifted.]
But I want to note Wisconsin and also Iowa to a lesser degree, with those two being the states that may have flipped the most in aggregate numbers and also had the most counties that flipped. One does see the Bernie story in Kenosha county in the southeast corner of WI, Orson Welles's birthplace, and old and depressed industrial town. But the largest area of county flipping was in the rural southwest, lots of counties, with this spilling across the river into northeastern Iowa. This is almost like the hot spot of the whole country, where an entire area pretty much just flipped en masse.
I am tempted to blame Scott Walker as bad governor producing a very poor economic growth record in WI, which got blamed on the national Dems. But I am not sure, as this does not hold for IA. In IA there are a lot of evangelical Christians, but less in WI. That is an old Progressive stronghold of Wisconsin (the northeast being traditionally conservative, where Joe McCarthy came from). GOPsters might say that it was popularity of Walker that helped Trump, although in general the GOP in WI did not like him.
Bottom line, I really do not know.