It has been pretty clear that Rmoney has been angling to get back into politics since his 2012 loss. So this is a pretty smart move on his part. He's not really from Utah, but he's rich, the "carpetbagger" charge has never actually lost anyone a single vote, he has positioned himself as an anti-Trump Republican (never mind what he was doing 14 months ago--ancient history!), and the state is possibly the most anti-Trump of the red states.
Evan McMullen would have more credibility as an anti-Trump Republican candidate (you know, because he actually takes positions opposing Trump), but I don't think he's running. (Wikipedia says he is not and that he endorsed the Mitt the Twit, but the articles it cites don't say either of those things and I can't find any other evidence for them). Also, while McMullen gets a lot of national attention, I have no idea how well he is liked in Utah. Before the 2016 election, there was talk that McMullen might win the state, but he ended up with only 21% of the vote, less than half of Trump and less than even Clinton got.
I predict Mittens will win the Senate seat and that he will be like Jeff Flake, giving speeches about the dangers of Donald Trump while voting for him 98.97% of the time. The only silver lining is left blogistan will get to pull out all those dumb jokes and nicknames we developed in his two prior presidential runs.
