How did mayors of cities like Minneapolis, Portland become elected when they are so seemingly unpopular? is it true they are unpopular?

In the case of Ted Wheeler it's because his opponent in his last election was a straight up avowed member of Antifa. So at least a small majority of Portlanders don't support things burning down every night...not that Wheeler's going to stop it either, he just doesn't actively agitate for it. 

You have the truly stupid voter who doesn't actually do any thinking and just buys that which is promoted the most or you have politicians who, once in office can only think about keeping that office for they can't make a living any other way.

How's life in your trailer park bunker in Wyoming? 

Yup...all "least bad" candidates.    Seattle resident here and your only choices are far left candidate or radical left candidate.    There are no sane democrats who don't think there should be homeless encampments with piles of trash, feces and people shooting up wherever they please.   Police are quitting right and left.  These politicians have literally ruined these once very nice cities.

Thank GOD Durkan isn't running again, but that doesn't mean there will be anyone worth voting for.

least bad candidate?  or maybe the 'unpopular' aspect is only the leftists in those places trying to seize power and the actual bulk of voters approve of the mayors' policies ...